There are a couple members here from GA that may be good contacts. Look up member "Wren" -- he has experience working on these and helped another guy down there get one going.
There was also another member who used to post here a few years back, just after the forum was first established, who worked at a Volvo dealer in South Carolina (IIRC?) and had a lot of knowledge and experience.... could be worth a try tracking him down though I haven't seen him post in quite a while.
Looks like you're on the right track with things to check, I agree it sounds like something is not right with your D24, sounds too slow. Everything is relative here of course, since in absolute terms a NA D24 is a very slow car -- but provided everything is in spec, it should be capable of drama-free routine driving without breaking much of a sweat. Not fast off the line but should be able to stay with traffic no problem assuming you're using the gears judiciously, and (at least on a flat road) it should go down the highway as fast as you want. Driving at high altitudes or climbing big hills it will put you in the slow lane but most of the time it should not feel like a complete dog. I don't know what kind of mountains you have in Georgia (unless you're talking about the Georgia that has a capital in Tbilisi, not Atlanta
) but if you're saying you can't get it above 55 on a flat highway at low elevation, there's something off with it.
How long have you had the car and what's the maintenance history on it? Has it always been this way since you've owned it, or did it used to be stronger than it is now? Did the fresh fuel filter improve it at all?
After fuel and air filters, timing and valve clearances are a couple of big ones when there is a power and mileage concern, I agree with your idea to check those. If you do some looking around you'll find detailed info on those topics posted on the forum. (We're working on getting a good FAQ put together in the future but for now the search function is the best way....) Any significant smoke at startup or under acceleration? What color and under what conditions? How does it start in the morning? Low engine output is usually pretty simple to sort out but requires paying attention to the clues provided in the way it runs; the more details you're able to post here, the more ideas you're likely to get about how to proceed.
If you get to the point of needing special tools, many of them are similar to common versions available for 4cyl diesel Volkswagens which can be substituted with minimal difficulty and are easier to find, or tools can be sometimes found on ebay, or bought/borrowed/rented from members here. There are some older greenbooks available for free download in PDF form on the k-jet.org site -- you might find a D24 manual there.
To continue discussion of what you are working on or trying to figure out, you may get the best response by starting a thread in the "Diesel Engine and Drivetrain" section.
Glad to have you aboard!