Some points I would like to add to the post:
If we can rebalance the entirety of the T3 land stage, I don't see how it would be a problem to look at t3 mex's cost. Arguing that its too big a change and needs years of testing seems pointless considering the scope of balance changes we have done over the years. IIRC T2 land HQ is like 50% more cost then 2012, T3 land HQ is also significantly more expensive, T3 land units are much weaker (except for the campy sniperbots!), T4's have been changed as well with greater buildtime cost. T3 mex has stayed the same. Perhaps its possible that by systematically nerfing land across the board for years, this is making mex is stronger in relation?? Anyone?? Also, Mex adjacency on factories has been buffed since then as well. Another reason to eco.
Level of gameplay has improved significantly. Generally speaking, we can eco smarter and harder, and take advantage of mistakes more. Everyone has dozens of engis roaming the map for reclaim. The most effective use of reclaim after a battle is to dump it right into mex upgrades, then scale production/eco appropriately. Pushing into a base is hard because engis can spam walled PD instantly, then reclaim the carnage for more eco. I had a game the other day where I had to pull back 15 loyalists and 30 medusa from raiding a bunch of t2/t3 mex because there were enough engis there to spam T1 PD forcing away loyalists. That sickens me. 2500 Hp on a t3 unit designed to raid can not raid because of instantly spammed PD.
The ratio of cost per benefit of the t2 mex upgrade -> storages -> T3 mex is nearly the same, the only difference is how much you need to invest before payout. On a lot of wide open 1v1/2v2 maps its often a smarter move to force a t3 mex in your base then to keep upgrading t1 mex to t2 if it is outside of your base where it can be raided. I do not think that this should be the case that a wrapped t2 -> t3 mex is nearly as cost efficient as t1 -> t2 or t2 -> storages.
My opinion to the main argument brought up by Bennis is that reclaim values should be scaled down, and t3 mex cost increased. Both by a small margin, say, reclaim down 15-20% and t3 mex up by 10-15%. Start from there. See what happens.
edit - I also like everything tagada said above. Very good points.
second edit - Upon giving this some more thought, since FAF released, we have made significant changes to the following: T3 Land, T3 Air, T4's, RAS, T2 Land, Adjacency, Overcharge, Vet, SCU's, and are trying to make changes to SCU-RAS. There is really nothing else left to change at the T3 stage besides T3 mex. I think this is all the more reason that T3 mex should be reviewed due to how much the game has changed, and what the game might need currently to bring it to a better state.