Wow, I completely missed that (largely because I assumed a Go implementation could not guarantee constant time), sorry about that and thanks for looking deeper than I did.
On that note, how much of a guarantee is there in the Go implementation? I assume in most cases it's going to be constant time, but isn't that a little harder to guarantee when compared to the asm version? And if not why not just use the Go implementation everywhere for consistency?
The asm version is much more performant. The Go version is a port of the version in NSS, but the compiler is free to screw it up in various ways. Furthermore correctness is always a concern: cleverness in unsaturated arithmetic code can be hard.
On that note, how much of a guarantee is there in the Go implementation? I assume in most cases it's going to be constant time, but isn't that a little harder to guarantee when compared to the asm version? And if not why not just use the Go implementation everywhere for consistency?