Some of our clients are Return Path certified. The primary benefit, it does speed up IP warming. The other big help is when we move customers from one IP to another, as in new customers signings. If they get RP certified before they move, we can compress their IP warming lead time. I can't provide numbers on existing sends, no data. You would need to A/B the same sends over RP and non-RP IP's.
I guess it is expensive, but that is a decision for the clients to make.