Obviously i don't have the stats you have access to, but from my experience and from data websites, only galeforce Varus does well in the current meta, and the 30s cd nerf will probably affect him way more than those buffs.
The way i see it is that the mana buff wont change the fact that you have mana limitation early game and will not be enough to skip PoM or manaflowband so it won't really do much.
The ult buff is alright but you still can't use it as a primary engage tool in most cases because of how easy it is to dodge.
Lastly to comment on Varus performances, with a 1.5% pick rate and 60% of games being played lethality/AP, the crit Varus games are mostly just otp and mains who play the champ well enough to compensate for his many weaknesses