I have used Barnes and GMX in the field with no difference in terminal performance that I could see.
Where I see the difference is at the range, Barnes as a whole always seem to shoot better.
One of my processes for developing loads is to load Barnes and GMX's. Shoot the series for each and not only look at individual loads but the average of all the groups for the series. I have yet to have a GMX come out with the lowest aggregrate group size.
Just did that with two 338-06 rifles and the 185gr TTSX and GMX's with both Varget and TAC powder. TTSX out shot the GMX. The 210gr TTSX grouped better than than both the 185's. So I loaded up some 210's last night to take to Texas next week, and will take that same load to AK in June for black bear.
Have had the same results with a 300 H&H and 180gr bullets, with the TTSX shooting tighter groups..
The majority of my 270's shoot the 130gr Barnes T or TTSX better than the same weight GMX. I have one rifle that prefers the 130gr GMX.
