However, there is absolutely no evidence or commentary regarding their strength, so I think it's a stretch to assume that just because great power would have been ideal for a member of this group that each member of the group would have this characteristic is a jump in reasoning. It would have been convenient, but I think the point was that Lews Therin had his own private army of loyal channelers and it was access to a loyal group he could depend on that made them so dangerous. Even survival of the Strike doesn't say much for their strength - survivability has as much if not more to do with skill, experience and luck than just strength. You can't really generalize that individual members of a group have a characteristic when we don't know if that characteristic was necessary to be a member of the group. It would be very different if we were told that "only the most powerful young males could join the Companions". As it is, the most we can say is that they were very loyal and would follow Lews' orders absolutely.

P.S. I still think it is funny that we are told that only younger male Aes Sedai joined the Companions. If we didn't know that Lews Therin was heterosexual, I'd almost think he was robbing the cradle.