"My question is: if you put a 24L on a Mk2 and a future higher 30+mp Mk3 and compare same shot, will you really see such a visible difference in image IQ you put mainly on the web?"
Crjiro: you can't tell there will be 30+ mp in mk3. See 1dx. I don't think MPs would cause better quality for web in this case. Better sensor in terms of banding at lower iso plus lower noise at higher iso plus broader tonal range (see pentax) would cause better pictures. Assuming you would catch focus on your object btw