I'm going to have to disagree with the statements about boost spool increase using an MBC. If you experience that then the EBC wasn't tuned or plumbed correctly to start.
Spool time is not ever a function of the boost control solenoid. The only exception is rare and I have not seen it work with most Subaru applications. The exception is cracking the wastegate a small amount prior to onset of boost which can help, again this is rare.
Simplified answer...The waste gate should remain closed until the onset of desired boost or at least very close to it. This means that the waste gate solenoid is effectively out of the picture until boost target is close. Claiming a boost solenoid can speed up spool times makes no sense. If there is a resulting effect it was because of plumbing or tuning of the original setup causing the lag.
Spool time may change depending on how you plumb your solenoid (manual or electronic). Two basic options bleed style (stock way) and interupt style. Interupt style is much faster to respond and doesn't bleed air off exactly so less loss. There are other methods as well but the top two are the main ones used with our cars.