Okay, about to run some laser damage tests. 100 layers of armor on the targets, all lasers used are Far X-Ray, and firing will be conducted at point-blank range if you want to replicate.
If we're leading with the assumption that it's not pure single-column, I don't think it'll be pyramiding like missiles, despite what's been claimed--that sort of loss of internal damage doesn't match my experiences. I'm expecting to see something more along the lines of an isosceles triangle, with a single-column spearhead several layers deeper than the rest of the damage, with the column widening to three, five, &c. wide as damage increases.
As predicted, it's not pyramiding--you're gaining, on average, ~3 layers of penetration for every two blocks of spread, assuming efficient allocation of damage (that is, hitting a damage number right before the next spread point, which only the 10cm here did). This model is markedly superior to the missile spread, you have a lot less wasted damage. Just as with missiles there are sweet-spot damage numbers, but they relate to penetration efficiency in a slightly different way.
We know that missiles are perfect step-pyramids wherever possible, you only ever get one additional layer of penetration per two additional columns of width. With how the laser damage is applied (call it the Empire State Building spread or whatever), each two columns of spread can get you anywhere from one to four additional layers of penetration. I'd also hazard a guess that at extremely high damage numbers you might see even better ratios of depth-to-width, but for the most part nothing you shoot will have enough armor for that to matter.
However, the inconsistency of laser damage (due both to the player's inability to directly manipulate it as you can with missiles and the range falloff) means that trying to optimize laser damage seems to be mostly pointless, even if there turns out to be a neat set of optimal values.
Now that I've got a test game set up again, I might run some more damage model tests to refresh my memory. If you want to see anything in particular, let me know.