nighthawk15

Obviously my car has ZERO need for anything like this right now, but I do plan to build a fairly serious engine in the next few years and am starting to gather information. Anyway, I have seen a few guys with this setup and I think a couple others I can't think of the names of, but there's a few questions I haven't really been able to find answers to searching around. If some of you guys that either run standalone setups or at least know a decent amount about them could educate me a little on the subject I'd appreciate it! Thanks in advance!

1. What is the benefit to running a standalone ECU? At what point would you say these benefits make a difference?
   
2. I know it will vary wildly on the setup, but generally speaking what gains could be expected on a street able 400+ RWHP (M6) heads/cam/possibly nitrous forged 355 setup? I guess this question is really kind of similar to the first one.
   
3. Lets say if I were to run an LTCC ignition setup and basically want the FAST to run the engine while the stock PCM ran everything else, thus retaining all stock gauges, accessories, etc. and still being able to pass emissions testing (I only have to pass a scan. No visual or sniffer) what would this entail? I imagine I would need a harness for each? I know I have seen setups like this before but I have no idea how exactly they were set up.