Was there any research done to say if this actually occurred or not?
It is, of course, impossible to prove a negative.
However I consider it obvious that the sort of hardware used for botnets is not the sort of hardware used nowadays for hashing. Specialised SHA256 ASICs are not repurposed spambots.
During the era of GPU mining, I seriously doubt there was any reduction in home gaming. Merely many disgruntled addicts paying over the top for GPUs or gnashing their teeth at unavailability of bleeding edge GPUs. I doubt a significant number gave up world of warcraft etc.
Consequently I doubt the specialist market for Bitcoin mining hardware caused any measurable reduction of email spamming.
Consequently, I'd be a little surprised if any serious researcher thought it worthwhile them spending significant time carrying out rigorous research on this subject and reporting on it.