I've used Synchromesh, Mobil 1 synthetic, and RP's Synchromax.
The RP is by far the smoothest. It's really not even a close comparison. A couple of times I've accidentally went to 3 instead of 5 with the RP because there isn't any "nibble" on downshifts anymore.
There is a myth that the Royal purple will eat away at the TR3650 and even one of the mods on this site will swear he spoke with a Tremec rep who told him he same thing, but I've spoke with two people at Tremec and didn't get that story. I just got "we haven't officially tested it" and "I highly doubt it will eat away at anything".
I've had royal purple in my 05 for 80k miles. If it does eat away at stuff it's really slow at it :p