Interesting, I'm shooting a SX4 and I had about 164 pellets with the 3" #6's and 172 with the 3.5" #5 LBXR's with the stock factory extra full turkey (.670 caliper verified). I ordered a JEBs HH .650 and have only fired one shot through it a 3" #6 LBXR and I had 202 pellets in a 10" circle at 40 yards. These were all with temps around 30 degrees.
When I get some warmer weather I will try some 3.5" #5s and see what they do.
Also was you barrel fairly clean? Several guys have pasted on here that dirty barrels were their culprit for subpar patterns.