I have not used Hevi-X but I have used one of two boxes of Hevi-Metal I was given on mallards while field hunting. IAs noted, it is a combination of the two shot types which I feel incorporates the worst of both shot types. I killed birds but no better than with a single shot type. They weren't any worse either but, knowing the composition, I was very careful to pick my shots so I did not overly stress the weakest link of the shell. They are not something I would spend money on.
My experience with the other than steel non-toxics has been that they are not needed for 12 ga unless there are extenuating circumstances. An ounce and an eighth or quarter of steel #2 at roughly 1400 fps has done everything I needed when hunting pheasants. #3 steel is not far behind but I have found it lacking a little on going away shots at moderate to longer range. Too many birds were retrieved still alive and I didn't appreciate digging pellets out of the meat. It is a large reason I upped my shot size choice.
