My understanding of Rodinal is that it is not a good developer for pushing film; in fact, just the opposite -- it works best when the film iso is downrated somewhat.
Diafine, on the otherhand, is a very good developer for FP4+ rated at 250. I've used this combination frequently with excellent results.
I get very good results with FP4+ @200 in Microphen 1+1 (both in 35mm and 120) for 14 minutes, I would suspect rating it at 250 and developing for 15 minutes would give very similar results - perhaps a slight loss of shadow separation and a little more grain.
For my money, I'd have used Diafine. FP4+ @ EI 250 is about as good as it gets if you need the extra stop from this film. No muss, no fuss. Works every time. Keeps the highlights in check nicely.