No offense but sounds like you're taking your car to guys that only deal with local NA cars, or have no clue.
Take you car to a known import tuner, someone that has a good reputation, on these forums, and in your state.
Boost cut and speed limit cut have nothing to do with having a tuned ecu and tuning the car for your level of mods. Who told you your ecu has a boost/speed cut? The sever jerking is what i experienced on a cold night, when i had a std ecu, at 12psi and the ecu activated the boost cut, as it over boosted, due to the very cold air.
If your mechanic had an idea, there would be no way that he would run 15psi on a std turbo with a std ecu and smic. Is he trying to blow your turbo/engine on purpose?
- Lower boost to 10psi
- Purchase FMIC
- Purchase aftermarket ECU
- Increase boost to 12psi.
Do it in that order, otherwise you are asking for trouble and a big hole in your pocket.
If you want more power after that, you'll need fuel pump, injectors and turbo.
Then if you're aiming to make over 280rwkw, you may consider buying a forward plenum, otherwise your just spending money in the wrong places.