Best thing to do is weld two pieces together and cut it in two and see what the penetration is like it won’t be a lot
Did you say 150amp? I take it your putting a chamfer on the edges?
Have you put that internal bevel on the top plate as a challenge?
45 degrees although we all do is a little over try around a 75 degree is probably better push angle the odd thing with MIG decreasing the wire makes the weld colder increasing the wire to a degree will increase the voltage making it hotter
In really rough terms you want about 30 amps for every millimeter of steel thickness. So if thats two bits of 3mm black bar your playing with, your trying to melt in to 6 mm of steel. I know thats an exaageration but I suspect if you put your machine on full power, and had a play with wire speed youd get a decent weld.