Don't think the improvement in performance will be much. The value to performance aspect of photography gear has reached the point of diminishing returns IMHO. The current 50L is one of Canon's finest and is widely adored by professionals (Jeff Ascough and Jasmine Star to name a couple). It's still a rock star performer.
And the price tag on these would probably be $1899-$1999 which is a ton of money for a prime 50 IMO. I remember when the 35Lii was announced, a lot of photographers were giddy for it. Fast forward now, and most people would agree that the original 35L is more than enough to get the job done.
I was hesitant to buy the version II of the 24-70 f/2.8 as I didn't think you could improve on the Mk1. How wrong I was. Without going into reasons, so much better all round. It helped me to decide to go to the version II 70-200 f/2.8, again such a vast improvement. I also changed my 85 f/1.2 for the mk 2 version.
I cannot see any reason why the performance will not be much if Canon bring out a 50L II. Going on the examples given I can only imagine it will be brilliant. I always found the 50L too soft. Canon have lagged behind on their 50L. I truly believe it will be a winner and there will be many people out there that will consider $2k well spent especially when they get a good return on selling their mk 1 versions that can go towards the costs. The so called professionals who adore their mk 1's believe me will be trading them in as quick as lightning.