Well, I've spent a few days with the new power tubes. To try and answer my own question all I can say is, I don't know. It's hard to compare 2 year old Sovtek 6L6's with brand new Chinese tubes. Going off of memory I'd say they're very similar, I'm sure this why Mesa chose these. But why they're not using the Sovteks anymore is a good question.
The first thing I noticed however; was the physical differences between them. The Sovteks seem to be more solid. They're a lot heavier than the Chinese tubes due to the thicker glass envelope. The Sovteks just seem more road-worthy. Time will tell to see how the Chinese tubes hold up.