If a 40-ish year old guy not yet in peak shape is riding a 80's-ish steel anchor solo over rolling hills for two hours and averaging 20mph, how much faster could he go on a custom fit Ti bike with a couple of buddies on a flat 30 miler?

Although I have upped my average speed somewhat from last year (by about 1 mph), it seems like the gains are coming more slowly and I am wondering if I am at my peak for my level of mileage (about 1500 mi last year and about 2500 mi so far this year). So do I need to do interval training or buy a new bike?

Question "How much faster could he go on a custom fit Ti bike with a couple of buddies on a flat 30 miler?" I'm going to say somewhere between 0.5 and 5 mph.

Well, there's no way to tell how much your rollers are slowing you down, as the definition of rollers is, shall we say, imprecise. Signficantly hilly riding can easily take 3 mph off your average flat land speed. If the only change you made was the lighter weight of the Ti bike, your solo speed on the flats might change by less than 0.03 mph due to a two lb. weight change. Odds are, however, that you'd have faster wheels, a better position, better tires, etc. so that's pretty hard to estimate. Then, if you latch tightly onto the second wheel in a pace line, that can easily give you 2 mph. I think you can see the reasons for the wide variation in the estimate.

...go faster?

...increase your fitness?

...justify the cost of a new bike?

If all else stays the same, a new bike will allow you to go faster - but not significantly. There probably would be a big difference in efficiency and comfort depending on the componentry you select.

If your wondering if you still can make significant fitness gains without a new bike, YES. 2500 miles this year puts you on pace for something like 150 hours of training for the year. That might be enough to keep you in much better shape than your average couch potato, but you need WAY more time on the bike than that before you need to worry about exhausting your fitness potential. Most of the local racers I know average around 300-500 hours per season on the bike - and these are just local shmokals, not pro racing prospects.

If you're not doing any kind of interval training, then yes intervals will also make you faster.

vs. the TI wonderbike is very minor (a fraction of a MPH at best). The effect of 2 or 3 riders breaking the wind in front of you is huge. It could be 4-5+MPH. The effect of rolling hills depends on the the rider and the hills. For someone who likes rollers, the effects on average speed is small. For someone who does not like the rollers the effects can be huge. Plus, in a real race the heat gets turned way up on the longer hills in the rollers, and once your off the back your toast.

Structured or even unstructured training will do a lot more good than the bike. Get a couple of books on cycling Training. Freid has the "Training Bible" and "Cycling over 50". Carmicheal is popular now along with his books and training approach which is a bit different than Freids approach.

With proper training it can take 2-3 years to reach peak form. It is normal to hit one or two peaks in a year and then fall off some from there. With structured training you take advantage of the peaks and then go into a rest and recovery mode following the peaks. Then when you go into the next cycle of training the next peak should improve from the previous peak.

if a couple of riders infront of me started breaking the wind I'd probably slow down and hold my breath.