Definitely shoot for a target power. 15PSI is all relative.

Just for shits n giggles, I have seen 15+PSI on completely stock B18B's make great power. Some pretty decent gains can be seen by some headwork as well.

If I were you, Id boost the stock motor. Run 8-10+PSI daily, see how you like it. Thats a good amount of power, and with a solid tune, can be done reliably.

If you want more, or the motor gives, then go ahead and build it. I would at least boost the stock motor though before just going ahead and building it.