Zeiss is talking about reddish vignetting on the left side of the frame with M9. You won't like this - but you can fix it with CornerFix.
I use the f2.8, which is a fine lens. The price increase over the f4.5 was a lot less than I'd expect, & for that reason the slower lens seemed 'uninteresting.'
I use the 2.8 with external viewfinders (28 & 21mm) on both M8 & 9. If you wear glasses you can't see the framelines with M8, & you don't have 21 lines in the M9. Interference of larger lens & hood isn't a problem if you use an external finder. The CV external finders are OK – you don't have to buy the more expensive Zeiss ones.
I recommend a hood, because I got some flare, probably from M8 UVIR filter, when I tried to get by without one.
The 25/28mm hood works better on the M8 than the 21mm hood, but it vignettes a bit on M9 (or on film M).
Kirk