One of the biggest issues on the original VT was with the OEM solenoids. Specifically, the RCV control solenoid and the wastegate solenoid. We replaced them with MAC solenoids and they were ok, but those in the know switched to this one:
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Since I had one lying around, I swapped out the OEM RCV control solenoid for it. At first the difference was negligible, but after putting a few miles on, boost onset lag has decreased a good bit.
Pierburg is an OEM supplier for some Euro brands, such as Audi, BMW and of course, Volvo. Nice little quality upgrade that is plug-n-play and cheap to try out.