@dfort
Yes, I get the same conclusions watching closely to those adapters, and I think I'll not be able to modd it! Your suggestion is good, may be I can switch back to a regular EF mount by asking a replacement from Sigma, the only problem is that in Italy is very difficult to find Sigma spare parts... mmm... Anyway, thanks to Sigma that make APS-C lens that fits in EF bodies too!
Unfortunately, the modd is needed because on 5D3 the 3x window in crop mode is not really free-to-positioning around the frame: I don't know why (but I'm pretty sure there's an obvious technical reason!) you can't put the cropped window in the main center, but a little angled (upper-left, upper-right, lower-right or lower left), and doing this with an ultrawide lens it result in a visible distorsion...
Actually, it would be great if it would be possible to put the 3x crop window right in the center!!!