Hey Anesos I did a similar analysis and got different numbers than you. I checked 2351 chunks but have scaled everything out of 100 to make comparison easier.
Out of 100 chunks I found
15 diamonds from Y= 0 to 4
47 diamonds from Y = 5 to 8 (vs 16/100 for you)
0 diamonds from Y = 9 to 28
I found diamonds to be about 3 times as numerous from 5-8 as you did. It's still lower than what the configs should be generating but not quite so bad.
I also checked the amount of smooth stone between 5-8 to see if there was room for diamonds to spawn and 80% of blocks are smooth stone so that doesn't appear to be the problem.
Hmm, 256 blocks in a chunk layer, x3 layers to mine out to see everything in 4-8, and i need to only hit every third row...so roughly 500 blocks to mine per diamond, and need 10 to make machines. So 5000 blocks and...oh screw that, i'll just do 6k and go to the deep dark