I've also found R&F games useful for identifying glaring AI deficiencies. The first few games I tried, I usually had to produce a bunch of extra workers right after taking over. Or I found that my capital was on some endless quest for the Moai Statues (despite plenty of yet unworked river tiles). Tech evaluation was the last thing karadoc had been working on:
[...] In any case, the quality of the evaluations for individual techs is far more important that the depth of the search. So although I'm still tinkering with the way the AI looks ahead, I think the biggest improvements would be in how certain things are evaluated...
For example, currently techs which enable buildings are evaluated based on how much the benefits the buildings would give the player's cities... but it doesn't take into account having a "building backlog". I know as a player I'm far more likely to benefit from unlocking a building if I don't already have other stuff to build. But the AI doesn't take that into account. Similarly with units, evaluates units based on what they can do... but doesn't take into account how many of them they are likely to build before they are obsolete. If that kind of stuff could be evaluated properly, it would probably help the AI. (But I'm unlikely to get around to that at this stage. My work on K-Mod has slowed to near zero.)
As for Currency, I don't think that tech is as crucial for the AI as for humans because the AI isn't going to benefit nearly as much from gold trading. They're not going to use Wealth a lot either, perhaps mainly because they need to produce more military units than humans do. Even the extra trade routes ... more commerce is always better, but, due to reduced expenses, the AI is at least not in a position of being unable to expand further without Currency. But you're probably right that Currency is still neglected too much by the AI. Even if it's not as important for the AI, the potential for human tech trades is indeed problematic. I also have a note in the manual that says "Currency still seems to be underrated (and Code of Laws overrated)." That's in the context of having already tweaked the calculation so that extra trade routes in future cities are given more weight. I remember that the evaluation is actually quite straightforward, just predicting the extra commerce. One can always just experiment with iAIWeight values in XML, but getting the AI to value techs for logical reasons – which should then work well across all game settings – is laborious. So, yeah, short of some stopgap measure for an egregious case or two, I don't think I'll be working on that.
85 turns on Marathon ... that corresponds to just 34 turns on Normal speed as far as research is concerned. Yes, that seems too short. Looking at the formula, it seems that the Normal-speed chapter count gets multiplied by the mean of 100% and the Golden Age multiplier, i.e. (200+100)/200=1.5 on Marathon. Rounding to nearest should then result in 11 chapters. Let's see ... yes, that's what I also get in a test. I don't think I had actually tested this on Marathon at all. (Epic, yes.) Chapter length is 105 turns. Are you getting more than 11 chapters? Or fewer than 1250 turns in total? 105 is probably still too little. With 9 chapters, there would be about 125 turns; perhaps that's reasonable. Could get there by moving the Golden Age modifier closer toward 100%, i.e. (200+100+100)/300=4/3. There's a change I can manage to implement!

Though uploading an update is another matter still ... As a temporary measure, lowering RF_CHAPTERS_BASE in GlobalDefines_advc.xml from 7 to 6 should work.
Edit: I see that the civ count can have an impact too. With more than 18 civs, there might be more chapters. I should look into that adjustment too. And maybe 10 chapters of about 115 turns is better. The formula above would put Quick speed at 7, same as Normal speed, which seems one too many; length would then be only 40 turns. Epic should probably result in 8 chapters of 85 turns.