I have owned a 95 850 NA since new. One of the things that largelly affect performance and mileage is a malfunctioning hot/cold air flap in the air box. It goes largelly unnoticed until you inspect it. It steals about 15 HP and mileage drops about 15% too. One of the symptoms is an unusually hot airbox after a normal drive,and some engine pinging up to 2000rpms.

A small and quite succesfull modification that is easy to do and will give you much peppier throttle response is the replacement of the throttle body plate by the one from the 960 volvo. I have done it and acceleration from zero is much better. Mileage is not affected, unless you really use the extra womph at all traffic lights. If you are reasonably handy, you can do that in about one hour.
The plate can be obtained from Waltrip volvo and is quite cheap.

Good luck