To get the best out of any of the aftermarket ECU's you really need to set them up on a rolling road. This can be $$$. So it depends what your budget and expectations are.
If you want to tinker by yourself in the shed and make a few more ponies, ditch the efi, ditch the catalytic converter, replace with a carb and extractors. (there are plenty of how to's on some of the English Mini forums on what is required). If your budget stretches further look at headwork and a new cam (requires engine out for this).
If you like the fact that the Mini starts first time regardless of the season and enjoy tinkering with software settings, look at the efi setups and what is possible. From what I have seen they aren't making any more peak power than the carb Minis, just making it a more drivable performance option (smoother delivery of power and less peaky).
So in the end it comes down to your budget and what you want out of your Mini.
The easiest starting point for small money you can replace the exhaust and exhaust headers on a standard SPi Mini and you'll gain a few hp (not heaps) and it'll sound better
. If you decide to any of the other more expensive options you've wasted nothing as you can build on the performance mods you've already made.
