One subwoofer at full rms or 2 subs at half rms which is louder ?

There are enough variables here it s kind of pointless to try to predict, but just as a rough comparision ---

If they re identical subs there s a 3dB differential by doubling/halving displacement given consistent power.

They re all dual 4 s right? So the pair of infinity s couldn t be wired for 2 ohms. To be done correctly, they d be wired for 4 ohms, getting 200w rms.

The Diamond would be getting 300 at roughly half the displacement.

So the infinity s have a 3db advantage initially, but drop a dB or two due to reduced power.

Theoretically the pair at 100w each would be louder than one at 300w.

It would have to be assumed that the cone movement vs power of the two subs would be about the same (may or may not be true) and that they perform similarly in whatever enclosures they re in. Which is to say the guesstimation doesn t mean much. Besides, at <2dB difference it s hardly worth worrying about.

It depends on the efficiency of the particular speakers.

They can vary dramatically, some several times more efficient than others.

It's by no means impossible that you could more output from a single 250W than a single 500W die to differences in the speakers.