Bump rollover is hard to pinpoint, since you're also dealing with slacker head angles and world's better suspension forks these days...but for me it's all about the front. The trail out here have roots and rocks that are unmoveable and all seem to sit 3-4" out of the trail. Combine that with the twistiness of our trails, and you end up getting your front wheel stopped constantly until you build up a bit of steam. The 29er really helps with that, and that's why I initially liked it. There are other benefits, namely traction in the rear is quite impressive while climbing dusty trails (reduces spin out significantly).
When I converted my rush to 650b, I went from 26-650b with relatively similarly sized tires. There was a small but noticeable bump in rollover, but it wasn't as significant as the 29er. I didn't put a ton of time on the 26" version of the bike, but I really did enjoy riding the BRush.
I don't know that you'd notice as much of a change in the rear, and now you can get bikes that are designed to have decent geometry combined with big wheels, so the challenge is a bit moot. It would be fun to experiment with a banshee, as those are the most adjustable.