CheyTac82,
The information you were given by the Palma shooters is correct, but not totally complete. Yes, we use specialized 155 grain bullets for this game, but it's not because they're the "best" for this application. We use them because International Palma rules REQUIRE bullets no more than 155 grains in weight. We also use long (29"-30") barrels as you mentioned, because that's what it takes to get these bullets up to the 2950 fps range these bullets need to remain supersonic at 1000 yards. Given the choice, most of us would prefer heavier bullets of a higher BC, driven at a lower velocity. Better wind resistance, no problems in staying supersonic, etc..
As Culpeper said, the 168s were originally designed for 300M shooting, and that's what it excels at. You can use them at 600 with no problem, but they become a bit squirrelly when you hit the transonic region somewhere around 800-900 yards. All bets are off then, and you'll be much better served with a heavier bullet. With the 11.25" twist being discussed, he should be able to get away with bullets in the 185-190 grain range, no problem, and possibly up to 200 grains or so, depending on the make (length) of the bullet in question.