Really? Surely the limiting factor is tyre/road friction rather than brake efficiency? I'll grant it's probably complicated, with the initial braking being pad/rim friction, but fractions of a second later when you've scraped the water off the tyre/road must predominate?
having fallen off by braking in the wet, slight delay, nothing much happens, bit more, brake a bit harder - oh f**ck , on my arse !
As you've said, the limit is the tyres, but the problem with rim brakes is the delay in coming on in the wet - and if you then grab a handful as nothing's happening and they suddenly come on, then you're off.
That said, I've never used discs, so don't know if the delay effect is still there - certainly used to be on motorcycles ! And I don't know about the amount of faffage with discs either. Certainly motorcycle hydraulically operated discs are far from trouble free when used all year round.. Allegedly discs do work better in the wet. I'm not so bothered about rim wear to be honest, as rim life is adequate for my purposes anyway.
Conclusion - dunno - but might be tempted. And if I went bespoke, I guess I could get fittings for both types are change back if needs be.