For you I think you've made the correct decision to keep the M8 since you take 90% B&W.
Apologies in advance for the long post, but here's my own M8...M9 experience.
I recently sold my nearly-mint ( I was the 2nd owner) M8 at a 20% profit, to buy an M9. I absolutely loved the M8 and also loved shooting in IR with the B+W 092 filter noted in another post.
My M9 was bought at a suitably low price, with an 'unrecognised SD card' error following firmware upgrade, so I haven't been able to try it yet, but I suspect that I've been subject to a GAS attack rather than any logical decision about how much 'extra performance' I really need (as an amateur). A non-functional SD card reader is actually a very expensive fault, as it transpires that the card reader is on the mainboard that also contains the sensor and rear LCD screen. Leica have stated that they will repair my M9 under goodwill and this is a fantastic example of their good sales support (I am the 3rd reported owner and the camera was outside warranty).
I own a 5D Mk III and many alt lenses, and this investment prevented me keeping the M8 as well as seeking an M9, as my RF experience since last Dec has been so good with the M8.
I used the following lenses on my M8, in order of purchase: CV 35/1.4 Nokton, Jupiter-8 (Zeiss 50/2 copy), later also buying 2nd hand Summicron 50 and CV 90/3.5 APO lenses.
I also have a fine Contax G 45/2 that I had converted to Leica M (lives up to its stellar reputation) and have also since bought a Canon LTM 50/1.4 and the CV21/4 with a VL 21/25 finder. Note that the 21 doesn't require a finder on the M8, however I also have an M6. The latter two lenses were bought in a brief period of deciding to hold onto the M8 and not 'upgrade' to M9 (before my GAS attack :-0 ).