I own a few Tikkas, but none in the .338WM. However, I do have a Winchester M70 Classic Stainless in .338WM and I shoot PMC 225 Barnes X (the original barnes X bullet before the TSX came out).
I don't know how well my .338 would shoot the TSX, but it currently shoots under 1MOA with the older PMC 225 Barnes X.
People say that the Barnes X is a very finiky bullet - where it groups very well in some rifles, yet very poorly in others. Personally, the Barnes X shoots very well in all of the rifles that I have used them in (like 3 or 4). They also say that the new TSX is supposed to be less finiky, and thus more consistent, than the original barnes X - but I don't know.
Try out the TSX and let us know.
Zachary