I like my M261 kit but would like to shoot  the typical high velocity 22LR ammo instead of "standard velocity".  My understanding is that will damage the M261 unit over time.  Has anyone experimented with replacing the recoil spring with a stiffer spring to be able to safely use HV 22LR ammo?  

If the recoil spring is set up like the Ciener or similar,  contact Eric @ Lakeside Machine (the fella that is making the 22LR belt feed upper for AR/M16, and get a spring he sells for the Ciener, it is heavier, and needs to be cut to length, that might be your ticket to play with.

Many M261 shooters shoot HV all the time with few if any reported problems. I recall one guy mentioning he'd broken the positioning spring (the flat piece at the end of the unit) several times, I think he mentioned about every 5K rounds (he didn't say what kind of ammo he was shooting). When I read that I ordered a few spares from SARCO. That's a $4 or $5 part. So far I haven't broken the original yet.

I have several thousand rounds through mine so far, most of them Win Dynapoints (just barely subsonic) but a few hundred HVs of various kinds including CCI MiniMags, which are fairly hot for .22s.

Keep in mind that cocking the stock AR hammer spring soaks up a fair amount of energy. The Rhineland R22 uses AR fire control guts and it comes with a one-legged hammer spring, as .22 doesn't generate enough power to reliably run it with a stock hammer spring (it uses a 10/22 bolt).