HP = rpm x torque (approximately). More power from: mandril bent headers, cam (I have one too wild for road use), free flowing inlet, massive tune up then dyno time, gasket matching head ports, polish of inlet manifold internals (matte finish), raising the compression ratio, building the engine again and a tunable EMU. How's that?
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i