I don't know what egbp #'s you've seen but the ideal ratio is 1:1 boost to egbp so for 20 psi boost you would want about 20 psi egbp. The ratio is higher during spool up too. Subarus never acheive this 1:1 ratio and in reality more in the realm of 1.5 to 2.0.
Think about this then, a turbo running 20 psi with a 1.5:1 boost/egbp ratio would have 30 psi egbp steady state, but much higher egbp during spool up.
While its not crazy pressure I could easily see 30-50 psi forcing open the wastegate against a weak spring.