Three thoughts:

--You don't have have to be fast, as much as shifty on PR. Speed is better on KR, when everyone's running at each other from opposite ends of the field, Braveheart-style. But is a starting safety super-shifty?
--Isn't your safety going to be a little winded from defending 3rd-and-long before taking the punt? I'd think if you wanted to use a starter, you'd use an offensive one.
--It reminds me of when the Cowboys had Brock Marion as a KR. I wish I had paid attention to those returns; could have been fun.