I have a grate in my level but turrets won't (or can't) shoot through it. Any suggestions?

Possibility: func_brush it, set it to non-solid, then add a player-clip brush and a func_clip_vphysics brush over it. This will allow hard light bridges, funnels and I think gels through it as well, though.

You sir, are a life saver!

To be fair, I read the exact same advice just earlier today IIRC from some other thread in here.