you do have an aftermarket dump pipe yes? if not, boost will almost certainly taper off at higher levels.
also, do yourself a favour and f**k the solenoid off altogether.. all it does is get in the way (unless of course this is one of those controllers that stepper the stock solenoid).
Connect your EBC direct between your intercooler piping and wastegate actuator.. the stock solenoid is only useful for a stock setup.