Hibiscus are great back-budders, so cutting them back severely won't matter much. Though, if you cut back that severely, you should put the pot on a plant stand to get the remaining branches into a more direct-light situation (I'm betting you don't get direct sun right down to the pot the way it sits now - but I could be wrong). You will also probably want to replant into a bigger pot. I'm betting that what you have it planted in now is completely root-bound.
Good luck!