Mullet bikes!?!

This won't help you much but...

I tried it on my YT capra 29er, I was fully convinced it was going to be a game changer... Bike rode well with the 27.5 rear wheel, but when I put the 29er wheel back on I didnt notice any real change to the handling, but the rear end was clearly a lot smoother over small bumps. So as an average height guy (5'10) I would say full 29er all the way.

I rode a couple of downhill bikes with a 27.5 front and 26 rear. They had a modified seat-stay to accommodate the smaller rear wheel/preserve geometry. When ridden back to back (same model and build of bike) with a 27.5/27.5 version, the smaller rear wheel felt a bit snappier in the rear and was more "fun."

In the end, it is totally your preference, neither way is "right." Build a bike that is going to stoke you out and I bet it will be the best bike you've owned!

Great option for smaller stature or short leg length riders. Best to put the 9er front wheel on a 27.5 bike to have the shorter chainstay for those riders. I liked the 9ers i tested but hanging off the back and jumping/drops made me nervous i was going to tag the twins. Bit more cumbersome moving around. I expected that a bit in the corners but it was more adjusting off jumps. None of the problems were crazy bad and i really liked the speed the bike carried and through the rough. First mullet bike purchase last year and its been the best of both worlds. Its a yt decoy. I wouldnt mind a touch shorter chainstay but its a super fun bike so no real gripes. The 9er front is confidence inspiring rolling through steep terrain that may have rock or root roll overs. Keeps rolling through the compressions. The 27.5 seems to have the ability to kick out when needed on the steeper terrain too. Its working well both on the dh trails and the bike bark jump trails. Id like to build up a guerilla gravity mega in a mullet for next year. Ill still be able to convert it to 9er front and back for longer xc or alpine rides. The devinci spartan 9er is one 9er i thought about doing it to as well. Short chainstay for a 9er. Easy enough to jack the fork to 180 to keep the bb up and/or an external bearing bottom headset cup

don't shoot me, but...
I have not tried a mullet setup on my Evil or YT, but I have on my levo eMTB. I set the levo up mullett style from the get go, thought it was "unreal bro", but I recently put the stock 29 back on and I 'think' I like it better haha. Granted, I had a 2.8 on the rear when it was 275, but it felt slower, more sluggish-it climbed better, but felt a bit slow in corners?
Point being, I've ridden bikes for 30 plus years, I notice setup changes across the board but I couldnt say there was a 'game changing' difference between the 2....granted, a bit skewed because the 'test' was on a pedal assist bike.