Would a breast reduction help mitigate the pain from chronic costochondritis? And how would that affect post-op care?

It is possible that having a breast reduction could help alleviate some of your discomfort from the costochondritis. The surgery should not aggravate it as it is done in a completely different layer of tissue (well above the muscles). The post op bra should not be constrictive just supportive. If it causes more problems that it solves then I would agree to wearing no bra but this would be up to your surgeon.

Breast reduction commonly and scientifically has shown to improve neck and back pain.  To my knowledge that is not medical research to show that it can improve costochondritis.  An examination with a board certified plastic surgeon may give you  better answers 

The weight and pressure from your heavy breast understandably affects the pain from costochondritis. At a current 40 G cup a very significant reduction can be expected. The bra we use after is more for support and not for compression after reduction and only need be snug enough to hold you whole healing, perhaps two weeks. Uncomfortable, maybe, but the result could set you free of chronic pain.