HAHA @ 40gallon drum!!
Im running 275s on the rear end of my S1 33, and theyre stretched onto an 11" rim id say maybe(if they make them, im no tyre expert) a 295 would go on it no problems, but getting an 11" rim into stock not even lipped guards may be a slight headache, maybe 11" with a +40 or +50 offset would do it, but i dunno what it would scrub against on the inside, like hit the suspension or something.
Plus I dunno if u slam 33s how the camber turns out, id say the height of your car would make a difference too?!?
Probably best to talk to a tyre shop, actually a few tyre shops.
U really need a list of people who are running fairly fat tyres on the @ss end of their 33 with stock guards to post up their rim width and offset. Change the title of the thread so ppl do list their sizes and go by that maybe?
Or u could go out to your car when the rain stops and measure up your current rims, plus the distance to the closest inner object(ie: springs), plus the distance to the edge of the guard, do the math and u have the answer