I've only built one HWK for 8" drivers and tested it for about an hour and rebuilt it adding 2" to the top and 4" to the bottom chambers, leaving the center chamber the same. I was very pleased with the results I got by the lower tuning.

By having your center chamber slightly tighter, you will have slightly less benefit from the center chamber and a slightly lower tuning. The center chamber tends to smooth out the double peak response common to 6th order cabinets.

The overall effect of being off a little, if I understand your question, will be minimal. The two peaks, one high and one low, will be slightly lower pitched tone wise and slightly bigger output wise. Notice how many times I said slightly? Turn it up and enjoy it.    ;D

Hey, exactly what I needed to know fellas!

I appreciate it!  I can't wait to play with it.