This question seems to get posted every few months. I'm not sure why you are asking
... because I guess that the 1/180 x-sync and 1/4000 shutter limitations on the 6d are not hardware limitations, but put there by marketing to set every spec in the sheet below the 5d3. And because I am still annoyed by the missing afma on my 60d.
Canon has not charged for firmware upgrades and I've seen no serious possibility that they will.
That's because all were minor upgrades or they had no choice to revive a camera line (5d2, 7d). They certainly aren't in the habit of continuing firmware development after release, but if they'd introduce a modular system that brought them revenue they might. If I was a Canon exec I'd introduce it, can't get worse than now :-)
While its possible that the 1/180 synch and 1/4000 shutter are firmware related, they are appropriate for the intended customers, those upgrading from Rebels.
I'd certainly not pay to change them. Not having any info about the design of the shutter, its entirely possible and likely even that its life would be reduced by 75% going to 1/8000 sec.
I did a check of my 40K images just now and found 61 images using shutter speeds over 1/4000 sec. Thats .0017 of them, Then, I viewed all 61 of them, none really needed that high speed, they were shot letting the camera pick the shutter speed and mostly user error.
Most buying into a entry level body are not going to need the high shutter speed.