I thought alot of them also changed the gearing while replacing the diff^^^
Drivetrain upgrades included a built differential (the diff in the AP1 is like the Achilles heel of the car) and an upgraded clutch and pressure plate. Was also going to upgrade the flywheel as well. Did I NEED those upgrades? No, I could go with the plug it in and go method, and many people do it that way and are fine. But it makes the drivability of the car really poor. The Comptech SC kit doesn't even have you upgrade the injectors, and in their case, you're running your stock injectors at near 100% duty cycle under full throttle close to the 9k redline. 100% duty cycle will potentially wreak havoc on the engine.
If I got a standalone, it'd be K-Pro. I used to think AEM EMS, but since K-pro came out that got moved to top spot. I'd need to upgrade to a AP2 V2 valve cover for the cam sensor and a new AP2 crank pulley for another sensor for K-pro, but the better tunability translates into more power and better drivability. Drivability is key to enjoying the car. I've looked into something like a VAFC, but the poor tunability due to the lack of tuning points is likely to make things worse.
A standalone ECU is not out of the question for a future upgrade.... just don't know when I want to invest yet another $2,000+ into the car. Yes it would unlock more power, but I'm not sure how much with my intake/exhaust combo. I've already put in an insane amount in the interior conversion, suspension, wheels, brakes, exhaust.... and I still have my front lip (LED project V2) and side skirts to put on.
and with the comptech I've heard alot of the guys just selling their old injectors and upgrading for a little more coin