The slow acceleration could probably be due to the time you shift gears.
Probably happens in the 4th gear when you shift too early (ie: too low a rpm at 3rd then shift to 4th gear).
The GC8 is a good car and depending on how many bunta's you have defeated, it may get hard.
i have 26 stars (bunta mode) on akagi and till date, have not gotten further than that with my dc2.

i am sure the game is designed such that all cars can defeat bunta 30 times for all tracks. some cars maybe harder, some easier.
so perhaps for your GC8, learn to master the timing of your gear shifting and also, try not to bump the walls or guard rails and you should be ok.
