Rich Merritt

The Helix looks like a brilliant, although hardly any in the UK so not had a play with it. Just being able to put it down without a stand is priceless. 
R

I think so.
This is what Metabones says (for the Ultra .71x Speedbooster)
"Like the revolutionary original Metabones Speed Booster® announced in January 2013, the Speed Booster ULTRA m43 has a magnification of 0.71x, and so it effectively reduces the crop factor of mirrorless Micro Four Thirds mount cameras from 2.0x to 1.4x.  However, the new ULTRA design makes very effective use of exotic materials at the furthest limit of glassmaking technology, and as a result is almost perfectly corrected for use with all fullframe SLR lenses regardless of aperture.  The Speed Booster ULTRA m43 will also work extremely well with many DX and APS-C format lenses provided the image circle of the lens is large enough.  Optical performance of the new Speed Boosters is so good that the MTF of any lens attached to it will be improved.  Even the latest generation of ultra-high performance SLR lenses such as the Zeiss Otus series can be improved by adding a Speed Booster ULTRA m43. (More information can be found in the press release here and whitepaper here)"
http://www.metabones.com/products/details/MB_SPEF-M43-BT4
You might find those lenses might vignette for part of the range but not all.
I don't have those lenses or Speedbooster and most of my Canon lenses are FF but my one Canon APSC lens (18-55 ISii kit lens) doesn't vignette on my M4/3 camera at all and that is with (Lens Turbo .72x apox) and without a focal reducer.      In fact, the lens covers FF on my A7s from about 23/24mm and up and covers APSC on the A7s at 18mm just fine (Canon's  APSC is 1.6x while others makers are 1.5x).      
I think it is going to come down to a lens by lens case as to what can work and what can not.
 
With the XL (.64x) Speedbooster, Metabones says-
"EF-S lenses require modification to fit or may remain incompatible even after modification (but third party DX lenses can be used without modification)
EF-S and third party DX lenses may not cover the full Micro Four Thirds sensor in still photo, FHD video (all cameras) and 4k (GH5) modes (but will cover 4k video mode on GH4, GX8)"
http://www.metabones.com/products/details/MB_SPEF-m43-BT3
 

I thought I saw a comment about why I am looking to buy an M43 camera when I'm unsure about the lenses available. The comment seems to have gone but thought I'd clarify anyway. 
The GH5, for the money, has an incredible feature set for video and is a great lightweight stills camera. I'd like to use it as a B-Cam for video and a second stills camera for the small amount of photographic work I do. 
I'm sure some of the native lenses are incredible, and thanks to those who are giving me advice in that regard. I think it might be worth investing in one, to begin with. However I have to think about the rest of the gear I use, not much point having fabulous M43 glass gathering dust at home if I'm shooting on an F5 or a C300. 
Thanks again for the help everyone. Keep it coming. 
Rich

Jonpais, you are right. I think I am putting too much emphasis on chromatic aberration. I also understand what you are saying about using light small glass without adapters, I am with you 100%. If I was going to ditch my Canon system and go solely over to M43 I would absolutely go native mount, but sadly I'm not in that position and I have to think how the GH5 will fit into my current work and what cameras I'll most likely to be using in the future. As you point out, APS-C is most likely. 
I also agree with you about matching lenses in post, however, most of my work is edited by others and often they do not have the time or skills to do a proper colour correction pass. 
Maybe I could sneakily get just one M43 25mm without the bank manager noticing. 
R
Hey Jimmy, I haven't as you've probably guessed. But looking on DXO it scores pretty badly for sharpness and CA. The Sigma M43 30mm 1.4 gets the best marks, and isn't that expensive. 
R

Thanks Noone for the Kipon info, I will have to check it out. I did a bit more research last night and I'm guessing the Leica lenses are Panasonic's equivalent to Canon's L Series, would you guys agree? 
Thanks for the video Jonpais, I had watched a few of his videos but not actually seen his one. I presume he's using IBIS but I can't see it stated in the comments. 
I noticed in the video comments that people mention vignetting if using a lens at 18mm on the Speedbooster. Could someone explain why this would happen, as I thought the whole point of the Speedbooster was that it takes an image from the lens and shrinks it down to M43 size?  Why would vignetting appear? Does this mean I couldn't use the Canon 17-40mm? 
There are three uses for the camera I'm thinking of...
If I'm using the GH5 as a B-Roll Camera on a video shoot I'd need to use Canon glass on a Speedbooster to match other cameras. 
Just shooting video on the GH5, I'd need a good focal range of fast visually matched glass, so all from the same manufacturer most likely. 
Thirdly, using it as a stills camera I'd like wide aperture zooms with fast AF.  
Again, thanks for all the comments. Really helping find my way through the rabbit hole I've found myself in. 
Rich
 

Thanks Noone for the Kipon info, I will have to check it out. I did a bit more research last night and I'm guessing the Leica lenses are Panasonic's equivalent to Canon's L Series, would you guys agree? 
Thanks for the video Jonpais, I had watched a few of his videos but not actually seen his one. I presume he's using IBIS but I can't see it stated in the comments. 
I noticed in the video comments that people mention vignetting if using a lens at 18mm on the Speedbooster. Could someone explain why this would happen, as I thought the whole point of the Speedbooster was that it takes an image from the lens and shrinks it down to M43 size?  Why would vignetting appear? Does this mean I couldn't use the Canon 17-40mm? 
There are three uses for the camera I'm thinking of...
If I'm using the GH5 as a B-Roll Camera on a video shoot I'd need to use Canon glass on a Speedbooster to match other cameras. 
Just shooting video on the GH5, I'd need a good focal range of fast visually matched glass, so all from the same manufacturer most likely. 
Thirdly, using it as a stills camera I'd like wide aperture zooms with fast AF.  
Again, thanks for all the comments. Really helping find my way through the rabbit hole I've found myself in. 
Rich