Well, sounds like the new 6266 showed up today at MAP

They are mocking it up now on a test engine outside the car. It is indeed the 1.0AR housing and after hearing this from Aaron at English Racing I feel pretty good about not going with the .84 A/R:
6466 testing (not quite what you are asking but similar)
2.3 VIII with a SMIM made 22psi at 4500 with a 1.15 vs 13psi at 4500 with the 0.82 T3
Volvo 2.5L we saw 34psi of backpressure at 27psi with the 0.84 TS. A more efficient Evo X would be even worse. Currently that same turbo went on a 2JZ (3.0L) in the same car with the 1.00 housing and was 13psi backpressure at 18psi of boost before we had to stop because of an injector issue.
It would be my opinion that the 1.15 is the best choice but since you are wanting more response than peak power I'd start with the 1.00. Worst case is you have to buy a new housing and then retune.
So he thinks even the 1.15 for the X but sounds like the 1.0 is a better choice for sure than the .84. So game on.