My 850R recently had a turbo replaced due to oil loss. It made a noticeable improvement to the way the car goes. However, I would say that the decline was more gradual than you are describing.

A common problem I've seen is blocked cat convertors (not necessarily on Volvos though).

Check the basics before deciding on the turbo:

- clean fuel injectors

- replace fuel filter/s

- OBD2 check for msgs

- air cleaner ok?

- spark plugs time expired/worn

- etc

I also noticed a big change after having the injectors done. The screens in the injector inlets were coked up with black gunge.

Since turbo 850s first came out, I have yet to see a bad turbo (failed bearings) on one , especially with only 90K on it, so I think your problem is likely to be something simpler like perhaps a hole in an intercooler hose or vacuum hose leak?

Under normal driving circumstances, especially with a water cooled turbo like yours, there's no need to idle to engine before shutting it off. The only time you should give that a thought is if your foot's into it first, like you just got done racing someone at full throttle, then I'd let it idle a minute before shutting it off.