Well I went to see it and the marks were not nearly as bad as I'd feared, so no worries about the front element. There is a mark on one of the rear elements (but not the drop-in filter), so I did some tests outside the shop.
This was an extreme test for flare, with a very bright light source and dark background. I certainly don't see any cause for concern here, with only a tiny bit of flare visible towards the bottom centre.
I took this one looking across the road at a display in a shop (so shooting through the other shop's window). This is handheld at 1/200 (no IS remember - I reckon the inertia of the lens holds it steady), using 5DII, so I could check the whole frame area.
And finally a shot looking down the street at a road sign, so focus should be good. This was at twilight, so 1/250 (handheld), f/4, ISO 12800, but the sharpness is very impressive, right out to the corner. I'm really happy, but it is mighty heavy!