I think we have to discuss what we want the experimentals to be. Here some thoughts of mine:
In pre engymod era t3 spam was very limited. You had to build experimentals to get enough firepower out in lategame. But now we do have engymod and can spam t3 units quite easy, so this role experimentals dont have to fill in, and are not that good at it either.
Also we could adress the veterancy problem experimental unit-balancing has. They get 10%*maxhealth*vetlvl each time they vet up. In summary a vet5 GC has received +150k hp. In 3599 and later their biggest strengh was their veterancy but now they face t3 were they just cant vet up and die utterly.
Now my thoughts:
Increase buildtime of experimentals extremly (factor 2-10) but make them more mass efficent. Either lower their cost of just buff them with enough health to survive some percy shells.
And change veterancy, that units either dont get free hp, or give them always +10%maxhealth, not like now vet5 50%. (without free hp the regen could be buffed big times).
This would make Experimentals in a later stage of the game than they are now, but beeing masswise way better than now.