I suggest that Sentry posts(and maybe guardian statues) get a regen upgrade as well. I suggest this because I noticed the sentry post tends to die a lot, and is not normally reached by the repairs in your walls. When playing invis mode a sentry post is essential, and it can be a pain having to kill already built towers to get to where the sentry post had been. So, maybe the "tower regen" upgrade could be changed to "tower and support buildings regen" upgrade, or you could had regen to the upgraded sentry post(recon post).
BLinkY -.o