Had 2 of 4 units show up that perform slow and can't seem to run any faster than 4.75TH. At stock 600 they performed very close to what you describe, they ran at a max of around 4.6.

I down clocked to 593 and now they run faster than when they were at 600.

The 2 units that perform nicely run at 4.8 all day at 600 but will run at 5TH if clocked to 625. Temp does not seem to ever come into play. They will only run so fast regardless of clock speed and ambient.

The 4.6 units will run at top speed of ~ 4.75 when clocked appropriately, the 4.8 units will run up to ~ 5TH.