I use an Altaros internal regulator and quickfill/gauge in my .22 S200.

I chose the the Altaros one because I did not want to drill a breather hole in my cylinder and nor did I want to extend the cylinder or move the weight bias forward.
I have to say it works very well and despite losing internal volume the number of shots has gone from 40 shots within 10 fps to just over 70.
You do have balance the firing valve, hammer spring and regulator pressure however to get the best results - but there's plenty of info out there to help you.
If it ever went wrong, I would happily buy another.

I also chose the Altaros quickfill/pressure gauge and while it works well and is accurate, I maybe would try something else if it ever went wrong.
Firstly, the probe hole did not come with a blanking piece to keep any dirt out - I had to make my own.
Secondly, the probe uses very small o rings that are easily damaged.
However, it was a while ago when I bought mine and they may have improved the design?
Otherwise I have seen ones with larger probes and rotating collars and I would probably try one of those instead.

The only other piece of advise I'd offer is to either buy the proper end fitting removal tool (or make one that copies it) as the cylinder end fittings can be bastard tight if they've never been off before...!

Hope that helps