For the mechanics of submitting a change, see
https://golang.org/doc/contribute.html.
You will want to run the crypto/elliptic benchmarks with
-test.count=10 (or so) both with and without your change, and run
golang.org/x/perf/cmd/benchstat on the output of both sets of
benchmark runs. Include the benchstat output in the description of
the change along with a brief description of the machine on which the
benchmarks were run.
I haven't seen your change, of course, but note that
crypto/elliptic/p224.go is a constant time implementation, and any
changes will need to preserve that. And note that while speed is
unimportant code comprehensibility is also important.
Thanks!
Ian