Custom barrels makers have pushed the envelope with chamberings for the Contenders before many times, not proved wrong maybe until a situation did so for them to stop chambering them. But they know no more about a Contender frames "strength" and what it can take, G2 or otherwise than we do from published data accessable for many years. They didn't get it from TC or the conservative custom barrel makers. So I guess it comes down to how much you trust them as custom barrel makers, because you may be betting your life on it.
I've posted the picture of one blown Contender 444 Marlin barrel and frame here many times. But many shoot it on the Contender platform with no problem at all as long as they stay away from hot loads. Yet another problem spurred by custom barrels chambered for cartridges they probably should not have been chambered for because the barrel maker can't be sure some nimrod won't drive it past what they advise is safe in it. The owner of a questionable barrel chambered for a cartridge with limitations in a Contender gets by only because he knows whats safe and isn't in it. But what about the next person to own it, or the 5 people after that? Regardless, the boundaries are there for good reason and pushing them will sooner or later cost a toll. You are far ahead of the game to do them on the Encore, or match them with wildcats that are safe with the Contenders rather than risk them on the Contender IMO.
My list of possible chamberings for the Contenders is something over 250 if I remember right, does have a few I consider questionabnle or at least limited to shoot safe loads, has few of the metric cartidges or their possible wildcats that also would be safe in it just becasue they are not in high use in this country or wildcats based on them have not been tried ye to my knowldge, and many safe wildcats that can equal or exceed some of the parent cartriges deemed as "Encore olny". Way more than enough to choose from for any task rather than risk the questionable ones on a Contender IMO.
Yeah I know, long winded yet again. But when safety is involved, which should be every shooters top priority, there is never too much said about it.