The GX grind is a good compromise and conservative enough to work well for the street in an A15, will have more bottom end than an a12gx due to capacity increase in engine but will rev very well up top unlike most A15 stock grinds. Then you can do as mentioned above but higher compression is a must along with a lightened flywheel will allow the gx grind to come on song sooner and punchier. Some quad 38mm bike carbs on a custom manifold and some well designed headers will be like night and day in response, sound and performance. If you want more after that then you are in for big $$$ as its easy for the first 20-30hp but beyond that is an art and science few are able to succeed.