I had my 16-valve B234F for years and had very similar experiences. Mileage was never great; especially bad in city driving, somewhat better out on the highway when the lock-up torque converter would kick in (AW72L automatic trans).

You have been pointed in the right direction on a number of components. I wouldn't throw a lot of expensive parts at your problem until you've done some diagnosis. There are checks for the fuel pressure regulator, engine temp sensor and O2 sensor. I believe you'll find these all described in the FAQ. New distributor cap, rotors and ignition wires should be considered routine maintenance items -replace when visually fouled or worn or when you can't remember the last time they were replaced. I recall a Bosch distributor telling me the original coils used with the B234F were known to be slightly weaker -the part numbers did change at some point. I did upgrade it when I put in new ignition parts, but I really don't think it made much of a difference.

After that, I think a lot of poor mileage with the 16-valve is the nature of the engine and different driving habits. There's a great tendency to be a bit of a lead-footed driver with the B234F. Although of similar horsepower and peak torque rpm compared to the B230FT, the torque curve is very much skewed to the right. Basically the engine doesn't adequately breath until you're up over 2500rpm. A manual transmission is a much better match for the 16-valve, but few were sold. To help compensate for this, the gear ratios for the automatic were changed with the AW72L, giving it a much higher gear ratio for 1st (with a large gap between 1st and 2nd), but they did add a lock-up torque converter to improve highway mileage. My point in describing all this is that a heavy-footed driver will cause the engine to hold on to 1st gear longer, obviously dropping gas mileage every time you pull away from a stop as is common in city driving. It is also easy to have it downshift into 3rd or even 2nd when you boot it for a pass. You'll obviously get a lot better gas mileage if you drive it like a small limo and never take it much over 2000rpm.

That leads to my second point. You've observed a lot of black fouling on your plugs. As long as it's not too oily looking, I would call that the norm with the normally low revving 16-valve, especially if you have an automatic, do mostly city driving or have a light foot. I had two solutions, and they did make a difference. One was to make sure it got some long high-revving highway runs -sometimes called an Italian tuneup (we're talking racing through the hills, so it's not meant to be a slur). That really seemed to help clear its lungs. But the basic thing I did to make all this less necessary and to improve performance was to go with a slightly hotter spark plug to help burn off deposits. I did a post on this many years ago, but I can't find it with the search engine (it may have been in the old archive "forum" which is no longer available), so I'll repeat myself here:

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B234F plug selection:

In the first year (1989), Volvo called for a different plug for the B234F, p/n 1367529 ("=Bosch WR6DC" right in the book), a colder plug than that used with the B230F/B230FT. In subsequent years, Volvo got smarter about B234F deposits and called for the same plug for all the B230F/B230FT/B234F engines, p/n 270.146 (closest to a Bosch WR7DC). Unfortunately, some auto houses incorrectly continue to cross reference the B234F to the colder WR6DC. So first off, if you go with a standard Bosch plug for a B234F, use a WR7DC (now a WR7DC+), not a WR6DC. Now many (including me) generally prefer NGK plugs and/or hotter plugs, so I'll continue.

I'm now going to point you to heat range info at sparkplugs.com then click to enlarge the heat range conversion chart at the upper left. Notice that Bosch plugs go higher in number as plugs get hotter, whereas NGK is the opposite, going lower (it's some kind of American/Asian/Euro thing).

In the case at hand, note that the standard Bosch 7 is lumped in with the 6 heat range, overlapping the NGK numbers. If you interpolate, a Bosch 7 would probably be an NGK 5.5, which conservatively gets rounded to the cooler NGK 6. You'll normally see a BPR6ES equivalent to a WR7DC in conversions charts.

So, if your plug tips are still running black with a Bosch WR7DC or NGK BPR6ES (and with a 16-valve in city driving they may well be) you might want to try the NGK BPR5ES, it's probably only a half-step hotter than the Volvo plug. These plugs (or fancier versions) shouldn't be too hard to find as they are listed by NGK for the B230FT on 760's, the B23F used on 240's and many other cars of the mid-1980's to early-1990's era, including BMW, Mercedes, Toyota, and Mazda.

This is exactly what I did with my 16-valve and it did make a difference -tips were then tan, bordering on black for mixed city/highway driving. Less deposits means less pre-ignition, means maximum timing advance, means better performance, means better gas mileage. It wasn't a huge difference, but it did help. And I always made sure I was using a quality 89+ octane gas, plus an extra annual injector cleaning with a good gas additive.

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Hope you at least found that lengthy dissertation informative.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now