The rs4 would get crushed by the m5 or by your car. No question… Not even close.
Pump gas to me is far more interesting. There are two ways to look at the dragstrip
A) the dragstrip is a measuring stick whee you want to see how your car stacks up against other cars, in It’s ‘natural’ form I. E. The way you drive it on the street. All your seats are in… Using pump gas… Street tires on street wheels.
Or
B) the dragstrip is a contest against the clock or other cars that are there where you do everything to make the car as fast as possible. The way I see that is if you’re using race gas, go all the way and take your interior out, and take your street wheels off and put some skinnies up from and drag radials with wrinkle walls out back and see what you can do. Hell take it further do an engine swap while you’re at it like the b5 guys are now doing to go crazy fast with vrt setups. You’re not really measuring what ‘a b8 s4’ can do, since nobody drives a b8 s4 like that, but if all you’re there for is the time or to win a bracket, go all out. Of course this would be a poor choice of car for that pursuit… There are far more effective ways of driving an ugly, interiorless car fast.
If you’re really racing for the fun of it and to get the best time, do everything you can. However if you’re trying to get a comparative benchmark for your four door luxury sedan in its true form, leave the seats in, use pump gas and leave your real wheels on…because that’s how you drive it 99.999% of the time. You don’t come across that random m5 on the highway and ask him to follow you home so you can pull all your seats out and put ms109 in…you just race, as is, on pump.