Hi Emanuele,
Just some anecdotal-type thoughts.
I recently got my Canon 40D modded/cooled by Central DS in South Korea.
Their website has a gallery of images taken by owners of DSLRs with this type of modification.
I studied it religiously before committing to the whole deal.
The images that stood out head and shoulders above many others were taken with the FSQ85 (by Maximo Ruiz) as the imaging scope - and I think a lot of the other pics used the 200mm f2.8L so a direct comparison is available to you.
One standout is Phil Hart's Rho mosaic image taken with the 200mm lens - wow!
http://www.centralds.net/en/index.htm
(click on 'Gallery")
I would hang on to the Baby Q - my 2c worth.
All the best with your decision.
Doug