My question is- did this enhancement raise the cap for exp as well, enabling them to reach 70tu from exp gains, or did it just take them to the limit of exp gains and leave them there?
Neither, but mostly the first. I tried to explain this, but apparently didn't do so well enough. Your 60-TU agent with a Bio-Enchancement will have 65 TU and another 5 can still be added through experience, the stat cap is now 65 + 5. Think of the +5 as a separate stat gauge that got added
and filled at the same time.
Well, the addition is more like 5-6 for a secondary stat like TU and 5-10 for primaries like Firing, if you do things perfectly and have a battle with 69 max TU while acquiring 11 points of relevant 'stat XP' (or just 1 in the case of secondaries like TU), but that's a bit beyond the point.
You can think of most commendations/transformations as giving you both 'exp gains'
and raising the cap. Which means you don't lose anything (except possibly the random field exp roll for select few commendations) regardless of when you do the commendation/transformation.