It's upgradable to a maximum of 323 attack. All new weapons post-DLC from sources other than the VoG are upgradable to 331 attack.
Bungie wanted to make the vex something special that would be more viable in the upcoming content so it got the attack buff of 323 before the DLC came out. Now that the DLC is out and every weapon drops with 331 attack maximum, we don't know if Xur will ever allow us to upgrade the mythoclast to 331, or if it will rot away eventually like VoG raid weapons.
the vex old and new is max 323 attack. That's the only vex you can get right now.
Ah I see. There are still some goals I have in the old content <rubs hands together>
Bungie did a great job in most other areas I noticed last night. I have all the equipment I need to hit L31.
Looking forward to stomping through VoG Hard mode as a L31. Hard mode sucked for my L29s
Btw, so glad I dont have to farm the world anymore for resources!!! Oh god that alone was such a great change should've launched like this