v-afc is useful to lean out you a/f mixture (most cars run rich stock) you can gain some power from that. you can also lower your v-tec point to where it makes the most power. They are pretty good power if you get it dynotuned.

If you do it yourself you can harm your engine. Running it to lean will mess stuff up and if you make v-tec come on too early the car will bog down.

The only bad thing about v-afc, and all other v-tec controllers i know of (excluding the hondata systems) only program the a/f ratio before STOCK v-tec engagement. i dont know for certain, but that's what i read the other day.

good luck