I have run 21 psi on the stock ignition without detonation. Now, you should note that I did that on 110 race gas, but the amount of timing advance you get at WOT will work for 18 psi provided that you use good gas and you don't decrease the MAP voltage (which means NO safc correction). I run 17 psi daily using 93 octane gas fed by the 8 injecctor setup on a completely stock ignition with no problems at all.

If I were you, I would follow what soulcontroller already said. I would stay away from the safc for any fuel correction at WOT. If you want to buy it to fine tune your idle and low throttle fuel delivery, then thats cool, but at 18 psi, you do not want your map voltage messed with. Running 18 lbs of boost on pump gas is already pushing it and any timing advance will probably start to cause your motor to detonate. You can accomplish your desired boost level using 440 cc injectors and the Vortech SFMU alone. Or, if you want more adjustability, and you don't want to mess with the MAP voltage, look into running 8 injectors using the 034eic, or the MegaSquirt. These setups will also allow you to retain the stock fuel injectors and pressure and give you the ability to dump more fuel than you would ever need.

Matt
1995 Eclipse GS
2.0L, S16g Turbo, 8 Injectors, 26psi

1998 Eclipse GSX
2.3L Stroker, AEM EMS Converted to Speed Density, FP3065 Turbo, 35psi , and so on...