What you might also want to consider is that the standard PLASTIC ended intercooler on the S2 will explode if running more than 1 bar boost over a prolonged period of time, my old standard alloy one on my S1 was very blown when i took it off and i was only running 13psi!!!
Your boost fluctuation may be a leak in your intercooler under extreme load, and IMO if an amal valve and actuator are set-up right it is far superior to a bleed valve, especially if you are prone to a spot of fettling