I really don't think the linked 35 IS vs 35mm L is a good comparison. The 35mm L shot wide open is not that good at all and if you are shooting the 35mm f/1.4L at f/1.4 continually your pictures IQ will suffer. Stopping down just slightly really helps.

At f/2.8 and the 35mm L and compare. The 35mm L is much better.

It is due an upgrade, one of the biggest helps this lens will get is that it predates the current coatings they use. The new version will no doubt produce better color. If Canon's recent releases are any indication the corners will be much sharper.

Since this lens is a work horse prime for many with FF cameras if Canon releases the fabled huge MP camera next year the 35mm L II will be released in advance.

This is one I would pre-order.