I have both the P365 and P320.

I plan on putting Holosuns red dot on both.

There are several side lengths options for both.

Has anyone noticed a big difference in shooting the longer vs shorter slide with red dots?

I have only put a red dot on my 365's, XL, the comped version, and the normal length. No difference whatsoever.

This is more of a general comment. When I shot irons I wanted more slide length because it improved my performance. Once I went to a dot the length of the barrel went from an aiming issue to a ballistics issue. And my conclusion is go short at least for carry guns, there is no downside.

For the 320 I don’t like the Full Size or Subcompact length slides. The Compact length just balances better for me. I have the carry setup. So compact slide milled specifically for the Trijicon RMR, WC Carry grip, Rainer Arms suppressor height sights, with 21 rounds magazines is perfection!

As for the P365 the Macro (no comp) standard height sights and a Holosun EPS carry will be purchased sometime soon.
By far the best P365 setup I have come across.

Specifically with red dots, No.