only at certain breakpoints. See Haste section for these
currently bugged, affects Healing Rain, but only as long as the buff is present. Will be consumed by the next healing spell. Will be fixed in 4.1. TotemForest did some PTR tests. According to those, HST is not affected, Riptide hot, Earthliving and Earth Shield are.
announced to affect everything in 4.1 (needs testing)
affects only the jump to the shielded target
check occurs on Riptide cast
recalculated for every tick
Didn't see this specifically addressed...
When I tested on PTR, Mastery was not affecting HST, but has been changed since. Stand in the Blue Circle did a more recent test showing the effects of Mastery on HST. I haven't hopped back on the PTR lately to verify that it's still in place.