AT mines are far from OP- two won't kill a sundy with even one rank of mine guard, and several ranks (not sure what point) will even save it from three hits.
It's 1 extra mine per level and two for the final level for 7 total. And I do think it's pretty much mandatory for anyone serious about pulling a Sunderer. Then again, I found (IIRC) six today that didn't have even the 30 cert rank 1, so effectively six free Sundy kills. I'm sure it will balance out in the future, as more Sundies have the guard or they can't be planted on stationary targets, but for now it's just an incredibly easy way to destroy a spawn.
At least when I'm doing my C4+rocket one shot kill to the backside of a tank they have a chance to survive if they see me. With the AV mines you can plant them fast enough that even them shooting at you normally doesn't matter.
Oh, and as for Extreme Menace kills from mines, easiest method of farming in a large facility (especially bio-labs). If you are about to lose it, plant every explosive you have around the spawn room within radius of each other, with a single anti-personnel mine in the middle. When the enemy take the lab they gain 1000xp, bumping their bounties nice and high just in time for them to rush into your minefield. I still need to play with the distances, but I think it would be possible to take out both of the infantry terminals on the ground floor with a single anti-personnel and two anti-vehicle mines. Keep sniping them through the shields until it falls to make sure they are hungry to kill you. Just be careful if you are spamming grenades out the doors that you don't fuck up a throw and set off your own mines by mistake. Not that I would ever do that...