Although I've never tried HP5+ in Rodinal, I regularly develop 120 Fortepan 400 in Rodinal 1:50. I get grain (which I don't find objectionable depending on the subject), but I also get incredible sharpness. The kind of sharpness that makes me feel like I could cut myself on the images.

Give it a whirl...with the right subjects I'll bet it's an amazing combination.