For myself, I haven't actually noticed that mashing much higher left me with a higher final gravity for 3711. Anything from 148 to 158, and it pretty much just eats its little heart out. But as you said, even with the really dry beers, they still taste very full, b/c 3711 produces a lot of glycerol.
The 8.5% imperial saison I brew won a competition a while back, and I got to do a pro-am brew for the GABF. The beer finished at 1.002 (both for the HB competition and for the pro-am entry). The judges' comments were all extremely positive, but all noted that it should be a little dryer and less sweet for the style. Even some of the comments on RB and BA mentioned this as well. For me, this was a pretty clear-cut blind-taste-test proof of how full 3711 can leave a beer tasting, even when it really dries it out to a very low FG.