No one really knows the true drivetrain loss every dyno reads different, doesn't nesscessarily mean drivetrain loss its just measured differently. i.e. Average Dyno Dynamic will read 310whp or so with a stock M3 that would be about 33% difference from the claimed crank hp. On a dynojet the stop whp number will be between 335-350whp depending on the machine. That could me a 18% difference neither one means that the M3 is really losing the power its just measured differently.
Based on my experience nothing beats a custom tune. What might work for my car might not make the same power for your car why who knows. But fact is every car is different when modded and exhaust from company A doesn't just mean company B's exhaust will make the same or more power it could make less and that can be enough to require a different tune. But even if all the cars are the same with the same stuff certain cars just react differently when they're broken in differently different fuel all kinds of stuff but I have yet to see a OTS tune make the same or more then a Custom tune.