@Jeyrsmith nice tree!
Powdery mildew: I treat powdery mildew with Natria (weekly spray, drenching top and undersides of all leaves), and that product solved the issue for me. Oddly enough (i still haven't figure this one out), powdery mildew only affects my shin deshojos in the back yard, and my red dragon in the front yard (my red leaf varieties), while my katsura, kashima, and koto hime that are up against the deshojos (literally leaf on leaf and branch and branch contact) have never contracted powdery mildew. (I am by no means encouraging you to begin rubbing your maples against eachother
).
Development: My concern is that if you were to chop the bloodgood off and keep the standard maple rootstock, what you are left with is a very young standard maple with roots that have not been grown with bonsai in mind. However, for very very little money ($5-10 USD) you can get yourself a standard maple pre-bonsai whose roots have been grown with bonsai in mind.
Would you be interested in keeping the bloodgood maple as you bought it, to be used as a landscaping tree? They are beautiful!
If you do decide to chop the bloodgood part off, don't forget that you can air layer it off and then perform your chop. There is no need to let your bloodgood upper go to waste