The FSR is a very nice first stage. I've never had the pleasure, as I dive XTX50/DST. Our second stages are basically exact replicas, except for the chrome faceplate.
When you mention the fart sound, do you mean fluttering? There's a Lake Hickory video about it. If that's the sound, many 2nd stages will have that, as it's just the diaphragm resonating with the body of the second stage at your specific breathing rate. I only notice it when I'm doing my buddy checks on the surface.
Regarding the overcompensation, I wouldn't know, never experienced that. I would maybe reach out to some others techs and see if they know about this, or even Apeks directly, and see what they have to say. Your LDS, after all, is just one tech with training dating back a decade or so.
If you can get the work of breathing issue sorted, I don't see why you'd change... unless you really have the 1500 quid burning a hole in your pocket, but there's better things to spend it on than yet another set of regs.