[v8volvo]

It sounds like the smoke screw is turned in too far. RPM hang / delayed return to idle is a common effect of that.

Could also be sticky action in the governor inside the pump, if the issue started unexpectedly and/or after the car sat a long time. That would be scary if so, since it could stick at higher RPMs too and lead to a runaway condition. This might be able to be cured by some of the flush products on the market -- Diesel Purge being a well known one. Many of us have even had good luck flushing with ATF. But you would want to be sure of this as the cause before going down that road, not just try it.

Refer to ngoma's questions -- we would need more background info to be able to give good answers.

NA pump installed on a turbo engine would certainly run the engine just fine, all else equal (that is, assuming a good pump that was installed and timed correctly, and no other underlying engine or fuel system problems). BUT the NA pump does not have the turbo pump's enrichment aneroid system that increases fuel injected quantity in response to boost pressure. So although the engine would run fine, it would perform like a naturally aspirated engine, not taking advantage of the extra power potential of the turbo. In short, not a great idea unless one is truly desperate and has no other option, as it would bring a major sacrifice in performance and driveability.