No I would not consider 150 grains of FFFG (3FG) a safe load in most all "Magnum" MLs. There are a few custom ML's that are supposedly rated for higher loads.

Ensure you verify for your particular rifle first.
Safe estimates for the 150gr "Magnum" 50 cal ML's:
Up to 150grains FFG Max
Up to 120-125 grains FFFG Max (Accounts for about a 15% powder reduction).

With 3FG, you will find that around 120 grains powder is about all you can stand for sighting in. That's of course if that much powder will even be accurate. The best way to tell is by starting out moderate at say 70-80 grains 3FG and see how things print. Then bump up if you wish.

Also, the 300gr XTP comes in two flavors, the standard 44 cal (.430"and the XTP MAG which is tighter fitting (.452" Black Sabot). You will find that the standard 44 cal XTP (Green Sabot) will eventually skip the rifling if you push them too fast.

m2c