I just bought my tc two weeks ago and first 200 miles it felt smoother than anything I have ever driven. Since then (I am not at 800 miles) it feels kind of rough when I accelerate and break. It feels like it has gotten harder to accelerate in the lower gears (I have AT) and doesn't really feel much different when I am going over 60 mph. The only change I had was that I decided to use shell v-power (91 octane) after the first gas tank. Obviously it sounds like that might be the problem. I was just wondering if anyone could explain to me why a better grade of gas would cause such a problem. I feel like taking it into the dealer to check out but I think I might first run on a few tanks of 87. Any info would be appreciated.
Thanks.

The tC's engine was designed to run on normal 87 octane. Using a higher octane fuel can actually be bad for the performance of the your engine. Cars today are designed to run on a specific octane that will minimize engine knocking (what you are probably experiancing) and are not neccessarily designed to run on fuel other than that.

Check out this thread for more discussion on this:

http://www.scionlife.com/forums/viewtopic.php?t=37555&postdays=0&postorder=asc&start=25

Not sure if gas octane cause you problem. I don't have a direct answer. I think, if you install some ground wires or voltage stabilizer, it will smooth out your engine and the acceleration will be easier.

I think it's just psychological and you just got used to your car's power. But when switching from my parent's 02 Accord and 03 Civic to my tC, I can feel a huge difference in power... until I get used to it again.