A replacement engine should have about the same power as the original and the "needs breaking in" idea is exagerated as I've replaced too many engines to think otherwise. It'll gain some extra power once the rings break in but most people would never notice it as it's not that noticeable. It's likely that they've either not re-installed something as it should be or another part has failed, maybe something as simple as a split vacuum hose?

You can try taking the car to another shop that knows your car, like a dealer or good, experienced FWD Volvo indy shop (if one exists in your area) but I'll be honest in saying that most techs don't enjoy trying to figure out what another shop botched, IF that's the case as I suspect. It can be time consuming and it's hard to charge someone a fair labor time (estimate) but it'd be to your advantage to tell the other shop what you've typed above so at least they'll be looking for something not properly installed, rather than some weird driveability problem if there's no codes available w/o a decent explaination.