I haven't tested those exact bullets, but have tested similar weight and caliber GS Custom bullets.
For example, the .338 cal 267 grain SP, I measured as having a BC that was around 30% lower than advertised by GS Custom.
The 232 grain SP was advertised about 20% higher than I measured.
The BC's advertised by GS Custom are calculated using software, not tested. It's my opinion that the drive bands are not properly accounted for in the BC prediction software, and that's what kills the real life BC of the bullets (drive bands).
One important factor with these long solids is using a fast enough twist. You want an SG of at least 1.5 to get all the BC that's possible for a bullet. With long solids, you can be looking at 1:8" twist to get SG's of 1.5+. Very few of the high BC solids will give you full stability and BC in a 1:10" twist, or 1:9.35" twist .338.
Take care,
-Bryan