We get questions similar to yours frequently.
Inherent in your question is the very false implication that bra cup sizes standardized are just like any other volume unit - but they are NOT. As you well know some bras fit you better than others. Each manufacturer has different standards (IE volumes) in cup sizes AND some actually fib - there is a VERY well known underwear manufacturer out there whose D bras are actually C's in other lines (Nothing like puffing things up...but we digress).
The question before you SHOULD be : Do I want a Breast Reduction or a Breast Lift?
A breast Reduction is done to relieve shoulder, neck and back discomfort (but a breast lift is thrown in in the process). The relief of the pain is what the medical insurers pay for. If your goal is a slight breast tissue removal and mainly a Breast Lift, you are asking for and should pay for -- a Mastopexy (breast Lift).
I suspect that if indeed you do have very large breasts (D or DD) - 500 gm should not be disabling and could still give you the range of a breast that C cup bras accommodate. The fact that BOTH your pain doctors and spine doctor ate advising you to do it tells us that you REALLY SHOULD. A Breast Reduction will have a HUGE improvement in your quality of life, mobility, exercise etc. BUT - there is NO SCIENTIFICALLY ACCURATE WAY to predict post-op bra cup sizes and although unlikely, you may end up with a smaller breast yet.
See a Plastic surgeon member of the American Society of Plastic Surgery (www.PlasticSurgery.org). You will not regret it.