My own rule-of-thumb for MTF cutoff is that I can expect "satisfactory" performance on a DSLR if the MTF is >70%. From the Canon book EF Lens Work III, a number of the Canon lenses meet this criterion wide open, with the 2x extender axially, but when we go out to, say, 12mm off axis (required by the 1.6x crop factor DSLRs) then most lenses drop out. The best behaved, wide open, with the 2x extender seem to be the 300/2.8, 400/2.8, 500/4, 600/4. From the MTF curves I'd expect that these 4 lenses should satisfy most people used with 2x extender if the subject is central, and anywhere if closed down one stop. All the other Canon lenses would seem to require even further closing down, reducing drastically their utility, IMHO. In particular the 400/4 DO looks like it would require closing down 1.5 to 2 stops, ie shooting at f11 or 16 to get good behaviour off axis. These views are simply from the Canon-supplied (some call them optimistic?) MTF curves. Personally I find them an excellent guide.