I don't think those downhill battles will be in GT4. Now, you can hope they do, since you are a bigger fan of them as I am (I said I liked "Initial D," remember?), but if they do get in, I don't know... well, I can tell you like this. There will be drifting, but I don't think downhill battles are possible in GT4. They can be done, but I don't think a lot of GT fans would want to slip and slide down a mountain. To me, if they want to do ANY racing involving hills of some kind, they'll most likely want to play Gran Turismo 2 with the condensed version of Pikes Peak. I don't think the downhill is as fun as going up, up, up into the clouds. So to answer your question about that, we'll leave that up to PD.
As far as CobraGT goes, 174 turns of drifting madness. Sounds cool. I think I even seen a segment on SPEED Channel last year whereas Marcus Gronholm (I think it was him), took his Peugeot 206 WRC around the Nurburgring long course (you know, part of the 24 hour race there). It was pretty cool. If traffic was halted or the city was shut down like the Nissan DVD called "The Run," then that Peugeot would tear ass across the streets. And since they are bumpy with jumps, you MIGHT think of "Nurb" as a road/rally course. Drifting? Sounds cool to me. I don't know if I would, though.