No its not unsafe to load a heavier bullet for caliber than normal , the trouble is that most of the time factory guns don't have the proper twist rate to stabilize realy heavy for caliber bullets so companies don't bother working up loads for them.
Now I'm not saying that you should take a 200gr Wildcat bullet and load it up in your 7mm mag with a load thats close to top end for a 160 class bullet , two reasons being that the longer heavier bullet has more mass and "generaly" has more bearing surface wich equals drag in the barrel.
I think that Wildcat bullets is making a 250+ gr bullet for 30 cal rifles ,it would be perfectly safe to get some and work up a load to shoot them safely but more than likely your barrel isin't gonna spin them fast enough to have them shoot well.