Well, so far I can't say that I'm super thrilled with how it performs in GT5. I don't think this is the fault of the buttkicker, though, as I experimented with it while playing Dead Nation and it was awesome. I actually had to turn the volume on the amp down significantly as the rumble was too violent on the same setting that I had been using for GT5.
The problem I'm having with GT is that I can't seem to find a balance where I can feel the more subtle things like rumble strips and off-road surfaces while simultaneously not blowing up the buttkicker when I collide with a barrier or another car (I'm getting an awful popping noise in these instances, kind of the way a blown-out speaker sounds).
After playing Dead Nation and seeing what the buttkicker could really feel like (not to mention at lower volume), I thought maybe the problem was the in-game volume level. Today, I tried cranking GT's in-game volume all the way to 120 so that I can keep the amp at a lower volume while still achieving the same level of feedback. This was a minor improvement, but it still sounds like the buttkicker is bottoming out whenever I collide with anything. If I turn the amp volume down to the point where I no longer get that popping noise during collisions, I lose nearly all other feedback from the game. I no longer feel rumble strips and the feedback from engine noise cuts out at very low rpm. Not a very immersive experience at all.
I have tried playing with the low and high-frequency cutoffs on the amp, and nothing seems to make any difference. The problem is always the same; when I find a level that eliminates the popping during collisions, I lose all of the other subtleties.
I currently have the buttkicker running from my tv's rca audio out (combined into a single rca which runs to the buttkicker amp). I have also tried running it from my receiver's LFE channel but I got the same popping and felt the range of response that I was getting was actually worse. Additionally, running off the LFE channel is annoying because in this configuration the buttkicker is affected by volume adjustments on the receiver. I wouldn't be able to play at lower audible volume and still get the same tactile response.
Anyway, if anyone has any tips on how I can configure this thing to my liking, I'd be very grateful. I've given up for the moment.