However, if you were in the late game with 10 cities, and you formed a permanent alliance with another AI with 10 cities, you would have a big boon to your research rate.
That depends on your view. I disagree because of following:
Lets say there are two empires, both producing 100 research/ turn.
There are 2 good techs which cost for simplicity 1000 each.
Scenario 1: both allied, tech costs increase to 1500 each
Player A and player B both research tech A first, then tech B
Tech A is acquired after 1500/(100+100)= 7,5 rounded up 8 turns and Tech B after 15 turns (in total).
Scenario 2: both allied, tech costs increase to 1500 each
Player A researches tech A and player B researches tech B.
Tech A and Tech B are acquired after 1500/(100)= 15 turns.
Scenario 3: not allied but playing together. tech costs stay at 1000.
Player A researches tech A and player B researches tech B.
Tech A and Tech B are acquired after 1000/(100)= 10 turns.
Player A and Player B trade techs.
Comparing:
Scenario 1 takes 15 turns to get both techs, but scenario 3 only 10.
So why should I ally? (Of course this works better in multiplayer or lower difficulties where the Ai actually accepts fair deals)