I am curious as to why you want to limit yourself to 26" wheels. If you're buying a new bike, why not at least consider the new wheel sizes?
Because there's nothing WRONG with 26" wheels, and I'm willing to forego the 6.34% theoretical potential efficiency gains in exchange for progressing with the wheel size I currently enjoy.
I'm not looking to smooth out my trails. I'm looking to become a better rider and I look forward to the rides where I can (sometimes) school my riding buddies on an 'outdated' 26er. If riding a 26er is slower, you need some extra combination of skill and fitness to keep up with riders on the bigger hoops. If I train/ride regularly on a 26er, I should accordingly, over time, become a better rider due to the greater necessity for finesse that those tiny wheels demand. Basically the same reason I stick with flats.
Additionally, I don't like being forced out of modern 26" designs by many of the manufacturers this year and am willing to go out of my way to support a brand that isn't making 26" wheels obsolete. (I recognise that bike technology changes over time - but, unlike a lighter derailleur or a less flexy chainstay, there is no distinct improvement with these newer wheel sizes). I refuse to support planned obsolescence and the disposable consumerist assumptions that support it.
Finally, I'm not at all convinced bigger wheels make a significant difference in performance, given Jerome Clements' and other's results this year.
All wheels have the same size contact patch with a given tire pressure - I'm certainly looking to run higher volume/width tires to be able to use lower pressures.
All in all, I'm excited about a new lighter bike with a good quality dropper post, and want to experience the difference while keeping wheel size constant.