Exactly. XL is all the way to go once beyond an extra 1/3 stop, you get more or less the same focal length at least, if not even wider -- 16 against 19 (FF equivalent):
(18mm * 0,64 = 11,52 * 1,28 = 14,7456** with vignetting, but)
20mm * 0,64 = 12,8 * 1,28 = 16,384** without vignetting (according to some reports)
25mm * 0,64 = 16 * 1,28 = 20,48** without vignetting
On Ultra without vignetting:
18mm * 0,72 = 12,96 * 1,48 = 19,1808**
On the other end, you can lose zoom range with:
35mm * 0,64 = 22,4 * 1,28 = 28,672**
against:
35mm * 0,72 = 25,2 * 1,48 = 37,296**
Nothing a pretty affordable 50mm f/1.4 (1 stop faster then) won't solve:
50mm * 0,64 = 32 * 1,28 = 40,96**
** FF 35mm format equivalent
:-)