I've done a fair amount of testing with Rodinal; I sometimes like Rodinal at 1+25 for the way it renders faces. Yes, your mids get pushed down, I assume because Rodinal is a "compensating" developer (I am no sort of chemist though, but this is what I've observed). Yes, grain becomes more apparent (tested this by enlarging the same crop of my test still-life very big with test prints from different dilutions). I've also found that developing a few degrees cooler has no effect on grain.
I can pretty much dial in Rodinal to get the same highlight rendering at various reasonable dilutions, in the 1+30 to 1+50 range, just by using a calculator. The percentage of developer increase has equaled the percentage of time decrease for me, but again, that's in a fairly small range where the developer properties are known. I have little interest in "stand" developing.
It's really not that hard to do some extensive testing - I set up a still life with hot lights or studio strobes; I use a small white board and mark the film, the bracket number, and the ISO's I am testing - I use a piece of tape to "tag" the frame for each ISO (I may have 50, 75, and 100 listed, and I move the piece of tape for each frame). This was, all the data is on the piece of film, which for me is a huge deal since it's easy to get disorganized. I aslo stick myself in these shots and use a wired release.
I shoot a bracket, shoot a frame with the lens cap on, remove the lens, set the shutter to "B", and stick a small square of scotch tape right on the film. Shoot another lens-cap frame, wipe the bracket number from the white board, and then shoot the next bracket. I can get about 6 brackets of 3 iso's to a roll of film. In the darkroom, I feel for the tape, and cut the film at those points and store it.
With one-shot developers, I'll soup a strip, and pour the dev into an airtight bottle - it was 'expecting' a full roll but only worked a few frames, I'll re-use it once. I blow dry the strip, determine max black (exposure time) in the enlarger from one of the blank frames, and go straight to a 4x5" print on MGWT with a 2.5 or 3 filter - I judge the neg by my final output, not on the light box, and all my test prints are exposed with the max-black time for that film & dev combo - I feel I've gotten most variables out of the process. Then I either blow-dry off the reel or grab a spare reel, and develop the next strip with adjustments based on the first strip - or if I've found a good time for 100 iso, I can see what I get for 50 or 75, and how differently the film/dev works at a different rated film speed. Of course, my temps, water, agitation and so on are all dialed in and consistent.
All those test prints and notes go into a binder I can refer to later. You can really learn a lot in an afternoon, with one roll of film.
