Id say your probably under geared, I was running 23/78 on a Epic 6.5 at Bury on Sunday
As people have mentioned if your struggling with understeer the first thing to do is glue the servo saver it can make a massive difference.
If you need any help or setup advise at Bury just ask and ill be happy to take a look.
If your looking at changing the servo take a look at the Savox range theres some awesome well priced servos