I'm not sure how likely a nerf for Tobi is but in the mean time if you really like him he's extremely useful.
Before any kind of nerf happens you'll probably be able to use him to get rewards in so many modes it'll be well worth the points you farm to get and upgrade him. Remember you also get 80 fragments from chapter 8 so only farm until you have 4 stars and 120 frags.

As for Kabuto he's very useful even in 2 and 3 stars. Unless you specifically need him to be stronger in some way you probably don't need too many stars on him.