In this case Valki has an interesting idea to reduce the mass cost and increase energy cost of repair. Another thing to consider would be adjusting the speed of repair to make it superior to ctrl-k and recycling the mass in. Even if it's not more mass or energy efficient, to get a t4 healthy and ready to fight very quickly again could sometimes be useful.
To go in a bit different direction, if we were willing to consider adding anything to the game, a land repair facility could be an option (similar to air staging). Anyone remember this from Red Alert? Not sure what would be appropriate mass/energy costs or speed of repair, but it could be interesting. I expect this might be difficult to implement, given the massive difference in unit sizes from t1 to t4s...but maybe just a very large building that can accomodate one t4, or 2-3 t3 units at once, 4-6 t2, or 10 t1 tanks at once? or various tech levels of repair facilities that have different capacities (similar to transports).
Repair does need some kind of buff though.