Aftermarket ECU may only be needed for an older car where you're swapping out an engine for a different engine. Modern day stock ECUs have all the computing power you'll ever need, along with the ability to support many variants of calibration. An aftermarket ECU wouldn't work anyway, your stock ECU is communicating with other ECUs and sending out vital data primarily on the high speed CAN bus to other microcontrollers which require this data sharing for your vehicle to even start.
I have an 86 Grand National, that still runs the stock ECU. However, all the calibrations reside on a chip and if I change injector size, for instance, I have to send my chip off to the vendor to have it burned or reprogrammed. Because my car only has the one microcontroller, the engine controller, I could swap in an aftermarket ECU with no detrimental effects. The most common for the turbo Buicks is the XFI, but I wouldn't see that much gain in performance to justify the expensive. However, running this aftermarket ECU does help in one way, it helps protect your expensive rebuilt engine. The fastest Grand National, with a stock ECM and stock heads has ran 8.94 at 150 in the 1/4 mile. Now if were to swap in an LS motor, which would make my Grand National worthless, if I was going to EFI, I would need an aftermarket ECU. The fastest Grand Nationals or Turbo Regals run aftermarket ECUs, but their motors cost $25k and they are also running either a Buick Motor Sports Stage II block or TA Performance block. You could use a stock ECM to run this blocks, but you won't be able to deep into the 7s in the 1/4 mile.
One thing that is coming in the future, which is going to make it harder or maybe even impossible without a getting the proper tools from GM to change the tune or code on any electronic control module is Cyber Security. People will say, oh, some will figure out how to get past security with no problem. Well currently all electronic control modules on vehicle have the security algorithm residing in flash memory and aftermarket vendors extract the image off the microprocessor and reverse the binary file into assembly or C code and that is how they figure out how to unlock security on a microcontroller. Once Cyber Security is implemented, the security will no longer reside on the microcontroller, but rather on a GM server and will only be accessible by GM Dealerships or GM Suppliers and that requires a non-disclosure agreement. It's going to it a lot tougher on aftermarket vendors.
So for the long winded answer.
