RE: Should I drop a GP Cub 40 for a different plane?
I hear your dilemma. Be advised that the longer you stay away from a once-started building project, the harder it is to pick it up again.
The long-wing Cub won't really do what you want; i.e., give you more spirited performance in preparation for a warbird, etc.
Here's my suggestion; continue to work on the Cub, but get an ARF to throw together and fly to continue the progression of your training. You would probably want to get something that can use the same gear as the Cub; so that would be a 40-size ARF.
The field is wide open here. The one caveat is that whatever you get should be a tail-dragger; but there is a ton of stuff to choose from out there. If aesthetics don't matter to you for this particular model, since it is nothing more than a "time-builder", look at the World Model's Skyraider II (I think that's right). For less than a hundred bucks, this is a real value if all you want is something that will fly. They are ugly, boxy, and very cheaply made; but they fly, and seem to hold up well.
Keep in mind the other advantage to having a "beater" plane (one you don't care much about). Your willingness to push the envelope is inversely proportional to the EDF (Emotional Debt Factor) of the model in question. IOW, if it is one you really like, you will NOT be inclined to try new things with it. And trying new things is how we learn & progress.
Anyway, that's my .02 on the subject. Good luck, and enjoy this wonderful hobby.