Canon's professional line has been lacking for years in the high ISO sensor performance. Their saving grace has been their lenses, which are still magnitudes better than the competition. That said, third party lensmakers like Sigma and Tamron have really been getting better in recent years, and the premium for Canon glass isn't always justifiable.
High ISO performance, and their auto-focusing systems are put to shame by just about every one else on the market.
While their lens quality and EF mount have probably been their saving grace, I personally don't find their lenses to be worth the cost. I left the Canon ecosystem for Fujifilm's X-Series cameras, and cannot believe the IQ their lenses have, across the board, and they're priced very competitively. I'm just a hobbyist at the end of the day, but when I can get an incredible portrait lens on the second hand market for $650, that rivals L-series glass, it's really really hard to justify that Canon premium.
Can you specify what you mean by high ISO performance? Do you mean read-noise or do you mean DR or something else? Canon is very competitive in read-noise and DR. Yes AFAIK they don't have dual-gain in their professional series, but the difference is not "magnitudes better" FWICT.
