Same size with F and P2; F feels a bit tighter, but I wouldn't go up half a size. I'd say P2 is definitely one of their most comfortable and versatile lasts unless you want something very sleek or are interested in wholecuts or monk straps without any toe caps or medallions - in those models, P2 can look a bit dull.
Had to go up half a size with U. It's a tough call because the shoes ended up being on the 'looser' side of a good fit, particularly in the instep, but I can see that they would be too tight in the toebox if I stayed TTS. But I guess I could see that coming: there are many reasons to choose the U or K last, but comfort is not one of them.
I've tried the R last once. I guess if I had to buy, I'd stay TTS, but I was thoroughly unhappy with how that last fit me, regardless of size, so I just crossed it out for future purchases there and then.
Anyway, I do think it depends on the model. Oxfords are going to fit differently than bluchers, single monks are more forgiving than double monks, and boots the easiest to deal with as long as they're not way too tight. The extent of (eventual) stretch, if you count on that when buying slightly tighter shoes, will depend on the amount of broguing and stitching. At the end of the day, it's a bit of a gamble.
It really all depends on your foot shape as well. I have a very long a narrow foot so the K last is extremely comfortable for me. I can't wear wide lasts like the P2. I'm not even sure if the F would work for me. I may give it a try in the future but for me it is really just k and U.