Dezod.com sells different AEM EMS'. Depending on the level of functionality, will determine the cost. Talk with Paul there, and he can give you the information you need to make a sound decision.
Secondly, You can push past 400 if you like. But that is already twice the stock wheel hp. The only people that are in that area are URD MKIII & turbo people.
400 whp is good for stock. Safe tune. Get internals if you want to push past that. Trust me.