btw,

does the shop do volvo service?
something as stupid as improper spark plug selection could do what you have observed. Plugs aren't hot enough reduced power. Carbon build up even further reduction. Pull a plug wire and see if they used bosch. Actually check to see if wires are done correctly. Not as familiar with the 850 as a should be to answer any questions, but i'm just trying to be helpful:)

This might be wrong, but..
Is the new engine timed properly
Is it the proper year's engine
if the timing is off or different year it may adversely affect power transfer and/or computer performance. I don't know the 850's well enough compared to 240's but in 240's this could make a difference. Maybe someone else here could shed some light on how to identify what engine was put in and if that was appropriate.

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.