I've played with B a while back, for the K3, and, yes, you hit UI restrictions pretty fast. The Touch has a slightly different UI, granted, but the same kind of restriction applies (it might in fact be worse)
I think I went with +25% / +50%, and @+50%, it does show issues in some part of the UI (and that was on the K3, which was a bit more 'text' based than the Touch...).
FWIW, and if you want to try it with ixtab's Font Hack, see this post
and this one
for said K3 stuff