I purchased the SCT after I did the exhaust, K&N hi flow filter and suspension. Bought it from Late MOdel resteration and they even made a tune based on my parts. Worked great, biggest improvement beside some HP was how mch smoother the engine ran. The other good part is that I get free tune upgrades with any change I do..Next is CAI and a larger Throttle body and spaceer
Both the Diablosport i2 and SCT X4 will do more than what the average gear head will need. As far as getting custom dyno tunes it largely depends on what the tuners in your area use.