I was wondering if there's any data available on whether the presence or absence of a rebate offer for the "full" version of ashcan games affected their sales.
From my perspective, that's the wrong question. Ask instead whether the presence or absence of a rebate affected whether a designer got the feedback they needed to take the game to the next level.
I didn't offer a rebate. I sold 40-ish copies of Acts of Evil at Gen Con and a double handful since, and I've had a great wealth of feedback and progress on the game as a result.
I think the only designer who offered a rebate was Kevin Allen Jr., and my sense is that he didn't get as much playtesting and feedback as he'd hoped. I'm not sure you can say there's a cause/effect relationship there though.