hmm... An idea would be to include a third, lesser power weapon down bottom mid with a faster respawn time. players then count the timer and and "clock it in" this will result in more player control of this area. a power up will give a similar effect. I personally would recommend a speed boost. That said, An Overshield could work with a 180 minute timer, if placed where your concussion rifle currently is placed, and move concussion to the lower wall, with a 120 respawn. Concussion rifle is a surprisingly good counter to Overshield I find. If the Overshield is placed up on that pedestal, 2 sightlines converge over it (exit of sniper room corridor and purple penties), meaning there is a large opportunity burn some of their Overshield, and you have alot of escape options in those areas, most notably purple penties. The marker for an Overshield will cause a convergence on that room, and promote map movement. it also gives a strong incentive to hold that room, giving your map that reason to hold a position it seems to need.
But, if you went down the Power weapon route, I would suggest a Scattershot with a 150 or 180 respawn, 0 extra mags, be placed where the concussion rifle is currently placed, and moving Concussion to the lower hall with 120 respawn time. something to make this environment more inviting.
Personally, I would go with a power up. Preferably Overshield. I hate it as much as you do, but it seems as though it could be the perfect cog in the machine to give the map a position that you really want to hold. The way your map is built, there is alot of ways to avoid conflict enough to let it burn away, a legitimate tactic I might add. The pedestal is in a perfect position for players to safely grab Concussion rifle to counter the Overshield while the opponent is preoccupied with said Overshield, which in turn promotes use of the under tunnel.