There is a thing on the G25 that I remember that my CSR totally fixes. It is when just cruising down straight, sometimes the wheel will start "pendulating".. Like it starts going back and forth on it's own, which causes problems and makes you fight it.
I am not sure if that still exists in GT, because I have not used a G25 on GT5 in years, or GT6 ever.. I'm not sure how the CSR fixes this issue, but there is none of that at all, and it is using exact same FF algorithms.
In those cases you just have to hold on to the wheel tight, and not let it get out of hand. It is annoying though. The G27 is a better wheel construction wise though. Better bearings, nice leather, aluminum structure. A thing of beauty for the price..
That's why it was tough to leave behind the G25 (similar to G27 as you know) in favor of a lower "quality" wheel. The thing is, the CSR has all kinds of adjustments that let you fine tune the rotation, spring, force feedback, "drift" setting, damper, deadzone (not used), etc.. And it makes GT6 (and GT5) a much nicer experience. The problem is the wheel is plastic and the bearings are not as bullet proof, and there's no leather, just small alcantara pads.
Anyways the point of that was to let you know that it may be that pendulum affect you are seeing.. And if it is still there in GT6 with the G27, you just need to run FF lower. I think 5 or 4 is the best bet.
Also there is a way to limit rotation to 720, 500, or something like that, 360, etc on the G27. I run between 650 and 720 on my CSR and that seems right for most cars. Maybe try to limit rotation to 720 or so and that might help.. Even lower for drifting. There's a thread around somewhere with instructions how to do it.
About MR cars tail "happi-ness".. Yeah, that is just all about learning to drive them, and it is tough in GT6 sometimes. Some cars seem almost broken, while other MR cars seem perfect. I think they'll be tweaking them in the future to behave a little more like you think they should, tail happy, but not impossible to drive. Like in real life.