I saw this thread yesterday on here and wanted to post a reply to this. I bought a 2008 Altima 2.5SL last night and inquired about the 3.5L and the octane level of gas. I was initially considering the 3.5 because I
WAS the owner of a WRX, which come stock with 227 HP.
According to one of their mechanics on site, and also confirmed by two other salesmen, it is not required that you utilized 91+ octane fuel in this engine despite the notifications of the manual. You
MIGHT see a minimal performance decrease (which I still would doubt) and it will have no negative impact on your warranty or affect the engine. What they did say would cause problems is if you buy 4 tanks of regular and then buy a tank of 91+ octane regularly. Switching back and forth between the gasolines is where they have seen issues.
This is jut from the horses mouth. I'm not making any guarantees. I drove a car which required 91+ octane fuel but only because of the presence of a turbo. A straight 2.5/3.5/etc.etc. should not require anything higher then your regular octane fuel.
Got the car for $20,164

Looking forward to joining this community!!