The second part of my post was perhaps not as clear as the first...
My additional space-terrain ideas are not meant as an *alternative* but rather an *addition* to speed reduction.
I was hoping the new *max* speed might stay higher than 15-18 (say 22-25 maybe?). I'm aware that, due to the exponential nature of the problem, a speed of 22 may be effectively just as impossible to deal with as 80+. But only Brad would know that for sure.
G.W. you are right in that the speed-limiting terrain ideas don't solve the AI speed problem directly. But, they might help the AI defend against a problem caused by it: The blitzkrieg.
In addition to generally slowing the ships down, if there were some defence measures that the AI could use, it might alleviate the problem and allow slightly faster speeds to managable in general. Not 80+ speeds but perhaps 20+ speeds.
Additional AI would be required of course but I thought it *may* be manageable. The nebulae would require additional pathfinding (around and through) and there might need to be a way for it to recongnize choke points and bias scout/fleet placement there, etc.
The starbase gravity wells would probably be much more challenging in that they are dynamic. If the AI detects one (or more) of it's planets have been conqured and it didn't even have any transports on it's radar (i.e. blitzkrieg potential), it could assume a large speed differential and include a slowdown strategy in it's defence algorithm. (Of course by then I suppose if it was a full blown blitzkrieg it would be too late anyway

)
I know that is overly simplistic but while additional AI programming would be required, It's not obvious to me that it would be beyond the scope of DA.
Regardless, thanks for the feedback. This game has captivated so many of us. The amount of time we take discussing it's nuances it testament to this!