I live in Denver--Mile high--5280 ft.

I understand that as long as the car is fuel injected that the injectors automatically adjust for altitude. I have an 850-R, so the turbo does help, but I also owned a 94 base model 5 spd, and never had any problems with performance. In the mountains at 10-11,000 ft all cars are going to have a tough time, with exception of the turbo. Remember though, at this altitude 10,000 ft, it is going to suck gas and a lot of it.

In town, I average 18.5-19. On the highway I get 29-31.

Hope this helps.

Use the proper octane gasoline. Don't haul excess cargo around with you, keep the car as light as possible. Remove roof racks. Maintain the recommended tire pressure.
168 hp is not terrible for the weight of the car, but if you are really interested in a performance volvo, get a turbo version.

is your glt a 95 model? if it is, it's a normally aspirated engine and not the light pressured turbo model...so...imho, there really isn't all that much to tinker around with (normal things like exhaust, air-filter) to help offset power loss from high altitudes. i could be wrong, but anything else (nos, etc.) would really cost $.