Upload & Sell: On
There's no newer Mark II glass in my kit, in fact the few 'L' lenses I use are not top of their class or are superteles, so I can't comment on Mark III T/Cs in that light.
However, with the 70-200 f4L IS, 300 f4L IS, a rented 400 f5.6L and the 100 f2.8L IS Macro (with extension tubes), I've found only minor center sharpness, much better corner sharpness with the Mark III T/Cs.
Mark II T/Cs are quite usable and I've not found any difference with AF performance with any glass, but many here on FM have experienced 'slightly better' AF with the Mark III T/Cs when combined with some Mark II lenses.
One note, if you still shoot film only the EOS 1V will work; communicate, with the Mark III T/Cs. I've a pair of EOS3's and my 1.4x Mark III does not 'compute', report, work at all...so I'll keep the Mark II's around until film is no longer.
Is the cost of upgrading worth it, yes was my answer, I felt the same when upgrading from the older 'most trusted/used' 100 mac to the new 'L'. Each upgrade eked out a bit more IQ and didn't bust my budget along the way...corn flakes, mac-n-cheese make for a tasty casserole; YMMV however