Significant loss of power after a fuel up...
 

I went to this place called Race Trac gas out in the boonies to fill up the IS300 (yes, questionable and stupid of me in retrospect) because it was pretty cheap. I took their version of premium gas and went on my business. Not long after at all, my car suddenly lost a ton of power - I would have to rev much more than normal to go anywhere fast and the car struggles tremendously to go up a hill unless I ease it into it.

I had the timing belt and water pump changed a few months ago, and had the oil and air filter done just two days ago. Are these symptoms of poor gas, or are there commonly other more severe issues at bay? It could be a whole host of problems - spark plugs, misfiring cylinders, etc. but the timing and coincidence of it coming up right after the fillup gives me hope it's just shitty watered down gas.

Could be your fuel filter being clogged since the gas was crap. Shame on you for putting fuel into your IS300 from the middle of Buttfuck Nowhere.

RaceTrac gas is fine. I fuel up there allllll the time and have had absolutely zero problems with it, whether it was 87 or premium.

It's actually a relatively big brand down in the Southern states.

Is there any other symptoms like over heating or detonation. (sounds like marbles in a tin can)

Is your gas cap on tight? Seriously.