To me it really depends on what your definition of power is?
Torque to get you moving,or power up top during a high rpm blast?
I see people comment that they would rather have a 383 to get the torque of a big block,but the 360 has a longer stroke than a 383. The 360 having a longer stroke and smaller port cross section will make more torque similarly equipped
Where a 383 shines,in my opinion,is its short stroke,big bore,and decent cross section for the cubic inches. They like to turn some rpm to make some power upstairs . Those 516 heads are not a bad piece either,closed chambers for decent quench.
I also think one of the advantages of the Magnum heads is the flow they get with the smaller valves than a similar flowing LA style head. This keeps the valve further away from the cylinder wall and helps keep from shrouding the valve as much with the small bores.
Keith