Hi
nope i think it will be ok, with careful focusing, even if u blow up the print big. Especially with 90 f2.8 or f4. there are even people who claim they have no problems with their 90 crons. but you have got to be very careful and critical about focusing.
remember, lots of pple have no trouble with their Bessa R or R2s focusing their 75mm or 90mm lenses and blowing up their prints and achieving good results. And remember, those Bessa Rs and R2s have a smaller Effective baselength (RF baselength x magnification) than the 0.58x leica M. Effective baseline is the true measurement of rangefinder accuracy. the 0.58x finder has a smaller viewfinder magnification (to cater for wide angle use) but it still retains the same RF baselength as that of the other Leica Ms, so the effective baselength is still higher and thus more accurate in focusing than a Bessa R or R2.
come to think of it, the Leica M CLE or Minolta CLE also has a smaller EBL than the 0.58x Leica. And it has framelines for 90mm. So obviously it is meant to take 90mm lenses.
the only problem with the 0.58x viewfinder is that the size of the 90mm frameline is very small. it is very hard to do accurate FRAMING, and very hard to see small subject details. yes, you can use a 90mm with it, but it will not be so pleasurable, unless one buys the 1.25x magnifier.
for longer focal lengths and critically focusing very FAST lenses, like the 50 f1.0 Noctilux, 75 f1.4 cron or the 90 f2 cron, nothing beats the 0.85x finder. Or alternatively, get an M3. Now that's a REAL viewfinder!
those new Bessa R3a may have a 1.0x viewfinder, but their RF baselength remians short, so the Effective BL is still not as high as a 0.72x or a 0.85x Leica M. this is one thing to take note. So far, the only alternative rangefinder that has a high Effective BL enough to match a 0.72x leica M, is the yet to the released Zeiss Ikon Rangefinder. i'm eagerly waiting for that one. it can rangefinder couple down to 0.5m! and those new Zeiss M mount lenses...... ;p