Drop rates on WBU don't improve as phases increase, but you will see an increase in the drop amount of T3 materials. It's small, but it's there.
The Booty Chests from Phases 20+ (blue chests and better) have a very small chance to drop premium comic cards and CTPs, so it's worthwhile to get at least a couple WBU bosses to that level.
As for the ROI? 450 crystals isn't all that much. It's 6 days of Stark's Stash (75x6=450), which works out to about $0.70. You can't get Hyperion to six stars for anywhere near that and right now he's utter trash. Still, that's 450 crystals you're not spending on something else just to move you closer to the (very, very small) chance of a P Card or CTP in a booty chest at some point down the road. If you can unlock the other bosses, do them instead & leave Corvus at Phase 9.
I did pay the 450crystals because I thought it wasnt worth the resources to rank up hyperion. I think its worth it for me since WBU is the main mode i play. The blue chests at level 20 have better rewards such as CCF and 1% chance of premium cards or CTP. Im parked at level 20 tho because i dont think its worth my time to go higher. However i did notice an increased drop of CFF buuuuut RNG could have just been good to me lately :)
I got a CTP of Destruction from a level 40 Corvus chest. and several times CCFs. While I can’t guarantee you are gonna get something good it’s a possibility. Also beating Corvus high stages is part of the fun of the game. I think to enjoy the game is more important than rewards, so I think it’s worth it. Unless you are F2P and haven’t gotten Magneto or Psylocke yet, I would do it.
Not really. CCF would increase in quantity but the chance of acquiring it is as shit it'll ever be. I personally haven't paid the crystals as I see no point in spending what could be almost a third of a uniform on a WBU boss, considering there are 4 more. From that perspective alone.
Now when it comes to books, I've no idea if it's worth it.