Fuel cut on a stock 07 ROM is set at 33.55psi absolute, so just under 19psi of boost. Your tuner needs to increase this value if he hasnt already.

I dont think you need a softer spring. Ive never heard of anyone boost creeping on a properly setup EWG, especially by 7psi over spring pressure. Hell I get creep on my 15psi actuator but only by a couple psi at 6K+. And this is on an internal gate too. My guess is that you either have a vacuum line hooked up wrong between the BCS, wastegate, and turbo, or there is a mechanical problem with the wastegate that is preventing it from opening fully.