Yes, just one pimp in the tank Phil, no pre-pump/in-tank lift pump or whatever you care to call it. Changed in 1995 i believe to just the one pump.
If the new filter has improved throttle response and power that much, arguably once the injector cleaner has done its stuff, it should be ok but it's worrying that the fuel filter was as bad as you describe.
With that in mind, i'd suggest also checking the condition and tension of the timing belt. The fuel filter should be changed every 2 years/24k miles (although Volvo often seem to work on 10k/year mileage) so it should only have been 5k miles old if the current mileage is 77k. It seems as though it hasn't been done since 48k miles (if then!) from your description so implies other maintenance tasks have been skipped or not done properly.
My old 740GLE had allegedly had the head gasket done with a new timing belt under a year before i got it. I noticed every so often, particularly when it was working hard, the performance would drop off. Knowing everything was as it should be, the only thing i hadn't checked was the timing belt and when i did, it was flopping about like a loose twig on a tree. That prompted me to also check the tightness of the cylinder head bolts. Glad i did as none were done up properly, it's a wonder the thing ran at all!
Ordered a new timing belt and didn't use the car until it arrived and was fitted, sorting the head bolts in between whiles. Transformed the car, performance, economy and response much improved and no more flat spots/lack-lustre performance when it was working.