I’d like to get some of the thickness of my p250 subcompact’s slide taken down, sort of what the 320looks like...
Any recommendations, experiences?

I understand that you like the looks of the P320's slide; it is attractive.

I am not certain I understand the thickness issue. In the photo below, the yellow line on the P250's slide duplicates the bottom of the angled cut on the upper part of the P320's slide. If you were the duplicate the P320's upper angle cut on the P250, the only metal you would remove would be above the yellow line and nearly entirely behind the ejection port.

I see your point...I guess it would take a little more aggressive removal, but that's basically what I'm wanting to do.