Anti-Tank guns OPP fire....One shot?

[J6A]

Given the 15 minute turn length, I think the lack of efficiency is due to lack of mobility.  Once the tanks find you, they aren't going anywhere fast.

If anything, I would say that AT guns in limiting terrain aren't spotted except within 1 hex (maybe 2 hexes for recon).  That's just an idea.  I've never tried it.

There's also a point where you'd getting into tactical level decisions rather than operational.  If I wanted ASL levels of granularity, I'd play ASL.  However, I'm sane.