I was trying to work out how best to compare laser diode modules together.  

You would think taking the reported power as a reference would be enough, but it is not.  First most modules report there input power, which is several times higher than the optical power coming out of the diode with little correlation from module to module.  

Next each module has a certain amount of beam divergence, that is the spread of the light coming out of the laser.  A larger amount of beam divergence will result in a larger spot after focusing relative to a diode with a smaller beam divergence.  To make mattes worse the beam divergence is not symmetrical, with one side having a much wider beam than the other.

Knowing the optical power and beam divergence (and beam aspect ratio) will get us much closer to being able to predict the cutting power of a given module and compare its relative performance.  On top of that knowing the rated max optical power of the diode will give us a hint of how much it is being over driven.

NEJE has quite a few laser modules but from the hints I found on the internet I feel that they are only using three different laser diodes:

// Nichia 5.5w, 450nm laser diode
// possibly Nichia NUBM44, 6W 
// beam divergence 9.7 and 60 deg
40W, 15w optical power
30W, 7.5w optical power
20W, 5.5w optical power

// Osram 2.2w, 450nm laser diode
// possibly PLPT5 450KA
// beam divergence 8.5 and 48 deg
7w, 2.5w optical power

// Sharp 350mw, 405nm laser diode
// possibly GH04W102GC
//  beam divergence 14 and 41 deg
3.5w, 0.5w optical power
2.5w, 0.35w optical power

The Nichia diode does not have a data sheet available online, but I found some info gleaned from laser diode enthusiasts.  I'm not very confident in the beam divergence calculations, divergence is measured at the 50% threshold and not the edge of the light, but it is safe to say the divergence is probably not over estimated by more than 20%.

We can see that the 2.5w, 7w, and 20w modules are driving there respective laser diodes at close to there limit but not too far over (assuming we guessed right on the module).  The 3.5w, 30w, and 40w modules are overdriving there respective diodes (assuming the 40w is using a 5.5w module).

I'm interested in the 30w module, it is overdriving the laser module by about 35%, we can probably extend the life of the module a bit by not running it past 75% power (certainly not for very long).  It does appear to have a wider beam divergence in the fast axis (the long axis) by about 25% to 45% over the other laser diodes, that will reduce the cutting power and make for more of an asymmetrical line formation. But it should still have a lot more cutting power than the lower power laser diodes.

The 2.5w module does have a lower 3:1 beam ratio (fast vs slow) while the others are close to 6:1, that should give it a more symmetrical dot when cutting or engraving.  The three element lenses that come with these modules have a tendency to square up the aspect ratio a bit, and that has a more pronounced effect the more out of square things are, so the difference may be closer to 2x worse with the more powerful modules rather than 3x worse.