Yes, I did mean that. I’ve done a few tests already - Colour only passes with direct only, low samples, no bounces on one layer, and light with GI, many samples, lots of bounces on another. To make it work in a single press of the render button, you need 2 scenes. Combine them in the compositor. Early tests: easy to set up, and obviously speeds up the 2nd layer, because you’ve effectively turned stuff off. The 1st layer obviously renders much quicker, because of fewer samples. How much time is saved? Some is all I can say at the moment, and it may be negligible. It obviously depends on how many samples you use in the 2nd layer, the more the more time saved. Turning on Z, normal, UV passes, etc will also see the time saved improved. (Put these passes on the 1st layer.)
I have tried the denoising on passes that you spoke of too. I know what you mean. Unfortunately the process works much better the more samples you do, and that means the less you want or need it! One way I’ve found to help the edges problem, is to use filters like the Laplacian on stencils. It works much better, but not perfectly, and the noodles can get very complex and time consuming to set up.
I realise you started this thread on GI only, guess I went a little off track.