I've seen a few Doom ports use a slope that is flat, and by making multiple sectors they make what looks like a curved slope.
In SRB2, this could be used in conjunction with some kind of script or invisible set of THINGS that the player will run on/over, and I don't even think it will need FOF's unless you want to stand on top of the loop.
So far, I have found that making ring circles in a Sonic Adventure type map and light dashing, you can make a psuedo-loop, which looks more effective with a texture that, from the sides, makes it look more like a curved loop.
I am still new to SRB2, but I have some minor experience with Doom, and I think Legacy should be able to do a slope effect that looks as good as the Doom port
ZDoom.
Although after seeing ZDoom, some will think of how SRB2 would do on it, but I think Legacy is the choice for SRB2 because it has that nice chasecam... (plus all kinds of work has already been put into SRB2 using Legacy..)