[coconut]

I had a suggestion of my own regarding sunderers, but it touches some of the issues you are mentioning, so I'll post it in this thread.

An hexagon would have its own "AMS energy", that gets depleted the more people spawn there. It recharges itself when it's not being used. Of course, each hex has actually three independent energy levels, one for each faction. The less energy it has, the longer is the spawn timer.
With this system, it's OK to have multiple sunderers deployed near each-other, as it does not bring any spawn rate benefit. The spawn rate is now controlled at the hex-level, not at the individual sundy-level.
It also prevents an attack or a defense to remain sustained for too long. If your side is dying more often than the other side, you should eventually lose.

In other words, my suggestion is similar to yours, but as I see it, slowing the timer on a per-sunderer basis can still be countered by bringing in lots of sunderers.

It has also a number of nice side-effects, e.g. putting an end to sunderer races, blowing up poorly placed friendly sunderers, and makes it easier to move up a sunderer closer to the objective without risking to disrupt the entire attack.