Not the 338, but I tried some of the Superfomance in 30-06 and 308. Tried 150's and 165's in each. All 4 loads were 100fps+ slower than advertised. My 30-06 handloads matched or bettered their speed and are MUCH more accurate.
The Superformance 308 was better. While slower than advertised, they are still faster than any handloads I've come up with by around 50 fps, and the accuracy was not bad at all. My handloads are a bit more accurate, but I'd hunt with the 308 Superformance loads I tried. The 30-06 loads I would not.