The problem is quite simple.
The T4 will produce the air for 350bhp at very very low RPM and as such requires the wastegate to shift far more exhaust gas to keep its speed low than the rest of the T series as they require a high speed to move the air required.

The T4 gate is often sub standard as it was never intended to move such low volumes of air. Its a crap turbo for the YB and Ford themselves had problems and had to sleeve the downpipe on the RS500 to make the turbine housing backpressure higher and thus artificially increase wastegate movement to stop the creep they found.

Many standard Rs500s still have boost creep and still hit the boost limiter when its cold.

Hope this helps.