No way overkill! I've competed in GT-Rs for the last 18months at our Vic SAU days (max speed is around 140, avg about 60-80) - and the GT-Rs go just as well as the 180s etc. Just gotta learn how to drive them and set them up really.
Good diffs, suspension & brakes will make the world of difference. Of course power will also help and either way if i was you - change those turbos before they fail and potentially take the motor out.
For a good khana setup if you put on say:
Garrett -7s / PFC/ Tune
Nismo Diffs & transfer case rebuild & a high volume sump
18s or 17s with good rubber
Brakes (perhaps the GB? Link in sig)
You'll have around 300rwkw, and it will be super responsive & nimble (well, as nimble as a GT-R can be) and handle like a RWD or AWD car as the diffs lock hard and are good for tight stuff to get the ass around and then straighten the wheels and bury the pedal and 4 wheels drag you out
I had larger -5s but for that type stuff on a 2.6ltr they are too laggy.