Keep Manfrotto QR RC4 system or modify to RC2 or Arca-Swiss (for slingstrap)
I purchased a Manfrotto 808RC4 head which I chose because of its solid design. I found out later that it's designed for medium format cameras (unlike my Canon DSLRs) making the RC4 quick release rather bulky for hand-holding.
However, the biggest problem is that I'm about to buy a sling-strap of some sort (most likely a Luma Labs Cinch 2). Unfortunately there's no means of attaching a strap to an RC4 so the way I see it is I either have to modify the head to accomodate a Manfrotto RC2 QR (they're smaller, hence more comfortable for hand-holding and can have a strap attached) or modify to another system (according to the Luma Labs link above they will supply an Arca-Swiss quick release for US$ 15 which the strap attaches to). Alternatively I assume I could attach an Arca-Swiss, Manfrotto RC2 or an entirely different quick release plate to the camera, then attach the other end into the RC4 quick release on the head. But I suspect that would cause some instability. I've found online instructions on how to modify the head for Arca Swiss plates, and I suppose then that getting an RC2 mount or some other system isn't too far fetched either.
I've tried to find out why Arca Swiss mounting solutions are so popular (and expensive) but really haven't understood why. My Manfrotto head/quick release system seems very solid, stable and well built to me, and additional quick release plates don't cost an arm and a leg, but I'd like to hear your advice on what I should do to solve the above problem.
Arca-Swiss style clamp got popular because of their excellent B1 ballhead. It remain poular because of the custom camera plates make by RRS and Kirk.
You will learn it after you use your system more. If you mount a heavy lens with the plate in the verticle position. The weight of the lens may loosen your camera from the RC 2 plate. But custom plate totally solve this problem.
Reading this reminded me that Manfrotto indeed has a which should solve that problem although I have no personal experience with that. However, there's still no way to attach a strap there as I was hoping for.
Apart from making it possible to attach a strap to an Arca-Swiss quick release system, are there other good reasons to justify the rather high cost of converting from the Manfrotto RC4 system?
No, all Manfrotto plate are subject to this plate twist problem unless you use 2 screws to secure your camera! But all DSLR only have one 1/4" mounting hole.
Having said that, if you are not using a custom plate to your camera. There are very little different between those mounting system. Lumalabs sell a thin Manfrotto plate connector system to go between the plate and the camera. That may be all you needed.