I have a Mid-2012 13" MacBook Pro with 10.8.4 watching in Safari and the fans don't spin excessively. Possibly knock the quality from "Auto" and/or "Best HD" down to "Best"? Stepping down to "Best" on a LAN stream results in CPU usage cut in half (~115% down to 60%) for me.
The other option might be that the plugin may not play well with retina displays--for testing purposes try changing your resolution on your retina display (or even attaching an external display and closing the built-in retina display).
Sorry, I've tried every combination of quality, resolution and using my Cinema Display as a main and secondary display, and it doesn't change.
The load on the MBP remains too high.
What it boils down to is this: I'd like to know of there are plans to improve the performance of the Slingplayer plugin.