Knowing the engine model, the hull and the rpm you run at would help. You're probably up against hull speed. H = 1.34 × √(L)
H (speed in knots) = 1.34x square root of the boat length. In your case about 6.5 knots. Unless you can plane, it takes a lot of hp to go above your hull speed. If you ran at 6 knots, your fuel bill would be about half.
If you want to go faster and you have a planing hull, then twin 2-300 hp engines should do. If you have a displacement hull, you won't get much more speed. Or get a longer hull. I have an 83' boat with 2 - 200hp engines and can do about 12 knots. Hull speed is 12.2.