I have several pairs of Sierras and have heard nuances ST RT's. Terry (TJHUB) has the HT2TL's and has heard my sierras and he has owned them as well, maybe he will chime in.
My thoughts,
For surround duty for movies, don't waste the $ on upgrading them, they are a fine speaker especially as a surround. If Multi CH music listening is your thing, timbre matching is more important although not as crucial as matching the front 3. I would say put the $ elsewhere rather than spending a ton to upgrade the surrounds to match, no offense to the Salk crew:)