I believe padding the timing always has been considered a bad idea. If you want to guarantee you won't exceed the timing, you probably have to take so much margin that your primitive becomes prohibitively slow. And I'm not even sure it would protect you against cache timing attacks.

Simply put, there are easier and cheaper ways to guarantee timings. Perhaps Haskell is even capable of harnessing them. (I'm thinking of the usual "no secret dependent branches", and "no secret dependent indices".)