Yes, go for the camber kit. You would regret it if you didn't if you are going to drop so low. You will go through a set of tires every month.

You will know how much you dropped by measuring from the ground up to the bottom of your fenders, adjust the coilovers the way CivicSiRacer described, and measure again. Then just subtract the second measurement from your first and that's your drop.

Honestly, if you move the collar on the coilovers two inches... you'll likely drop about two inches.

I've never used omnipower, but I've read reviews and people seem to like them.