They must be shooting all these .243's that you guys are mentioning at long range back East, or in other shooting disciplines, as I have not seen a .243 or it's AI variant at any F-Class match I have attended in Canada or the US.
I have however seen many 6BR, 6 Dasher, 6x47, 260, 260AI, 6.5x47, 6.5x284, many different 7mm and 30cal chamberings as well, but never a .243.
.243 is not the most efficient case design, if you want the speed of the .243 look at the 6SLR (6 Super Long Range) or the 6x47 Lapua as these are designed for long range accuracy.