Nice tank, and pretty awesome work with the driftwood.
I also use a needle wheel diffuser with a hydor pico powerhead. I just attached the co2 tube to the grill of the bottom. One important thing, don't place the tube too near the impeller, else it starts a siphon to the co2 bottle, causing yeast to end up in the tank(ha!, been there). Also use a separator bottle just in case.
About the impeller mod, I decide not to bother as the bubbles were already chopped up very fine by the stock impeller.
P.S. Noticed you have a bubble counter/separator, is that diy?