Not significant, but it will add development time. OTOH, since they are basically using a shipped engine it means they should have more programmer team hours to use up. Also, even before this announcement, JC had said they'd be looking to improve the renderer for DOOM while everything else in id Tech 5 would stay roughly the same so maybe there's some efficiency to be gained here.
Playing Devil's advocate here but if JC really meant what he said about the PC not being worth additional dedicated development time then this latest revelation seems to be a contradiction. Hmmm... could just be that they took a hit on sales but it seems a bit premature to base decisions on. Perhaps this topic surfaced during those id-AMD discussions.
I think for D4 the forward-looking way to focus on console development is to build a game for a very high spec PC. If they spend ~3 years on it they will probably want to port to the new 2015 console hardware. So, focus on high end PC now, and have a pretty good looking port for console launch time. I expect D4 will be fully DX11.