Wastegate diameter is pretty much the only variable that is directly linked to boost creep. A spring will do nothing to help creep. The boosts "creeps" because the wastegate is completely open yet still cannot flow enough exhaust through it to control the boost. Whether it "flows" enough exhaust or "is large enough" to equal the pressure differences is a moot point because this is the same thing, just worded differently.
Downpipes can also contribute (read HKS and stock designs)