Andrew-- NO I would not say FX-4 enhances 'grain' only the amount by which the film is outdated and this was outdated as I cut it from a 'Gift' Bulk tin that had some left in it -- it is all used up now. I have some left in that cassette that i will try my Home-Made 'FX-15 ( Acutol-S Formula) with next.
Andrew -- YES I find it does increase speed by at least half a stop but I have not used 'Fresh' film only 'outdated' so on Fresh it may be more. I WAS bought some 'FRESH' FP4+ for my Birthday -- I must try it !