I get to compare all my 29"er to a 26" hardtail I know like my own back pocket on an almost weekly basis. My buddy rides a VooDoo that was the first hardtail (non-rigid, non-FS) bike I ever rode. When I built it for him in 1999, and rode it around the block, it freaked me out and months later I had two of them. I got him into lending it to me for all races I had to ride until my own VooDoo HT was done. So, I regard the 26" bike as quick.
The 26" bike rolls like mad, but it always on a worn 2.35" Fast Fred rear and a 2.4" Racing Ralph front. Tires we in 29" land can only dream of.
But every corner, every downhill, every bump in the trail, every hill, the 26" bike is plain cumbersome compared to whichever 29" bike I brung that day. We always do multiple short laps, and tend to swap a couple times so he can give his opinion on whatever I changed to my setup.
Over a 16min lap stuffed totally with singletrack corners, I regard 29" as 20s faster, and this is with much slower tires even. No Powertaps to back it up, yet. I'm building a 700c wheel, some time.
I can't help but think, that if it wasn't the tires or suspension, Dave pedalled harder on the Dos where it was no use. Perhaps some more elaborate coasting cost bring the average power down by a couple percent without a notable time penalty.
Best to compare would be 2 rigid bikes. Nothing to adjust. Geometry and tires so similar that thay almost ride like the same bike.
Last night at the park, 1" of snow, I found a good way to compare maximum grip levels between 2 bikes. In the middle of a grass field, sat a perfectly circular labyrith. Grass around it was very smooth and consistent. Max speed when riding around the labyrith was limited only by grip, not power, not air drag, not rolling resistance. There was no cutting the corners.
Let's say you ride a couple of laps around such an object, rolling start, that would be a great indicator of the bike's cornering potential. Mostly the tires, I think. I was riding Nano's, front a custom cut one. I understeered. Buddy oversteered, with that worn rear tire. I was faster this time, we didn't swap bikes. In such a test guts is a factor, but if you practice at it, most everyone should come close to the bike's maximum.