What optic- other than the Romeo1Pro- will work on the XCompact without modification? Preferably a Holosun...?

The Delta Point Pro and the new Eotech EFLX https://www.eotechinc.com/eflx-mini-reflex-sight-mrs fit the footprint. If you want a Holosun or a Trijicon RMR you'll need an adapter plate from CHWS precision or some other adapter plate provider https://chpws.com/product-tag/sig-sauer/

Forward Controls Design has the best solution for RMR's on P320's with the rear sight plate.

https://www.forwardcontrolsdes...0-RMR-RSD_p_344.html

Adds a rear sight dovetail and you can get TD branded cowitness sights.