I don't think it's that OP. Sure the sentry can see a lot, but it's also visible way outside of the range in the porch of the other team's building, where they'd naturally be holding anyway.
If the engie uses the wrangler, it'd be easy to flank from below and take him and the sentry out. I'd compare it to putting one on the balcony in badlands, or on top of the train bed. Plus I'm always in favour for strong flanks.
Also, haven't you converted the tunnel into underground last into the dustbowl style? Get on it!
In my opinion, for this map to get into regular rotation, it needs to beat Gullywash - because they always swap out maps for whatever works similarly. This map already does the strong second hold that Gullywash has. So now you need to make it easier to take and push out of last than Gullywash, which is the problems that map had.