I think N2O should be added after a fully built motor...
For the price of the kit, a fuel pump and all the hassle getting it tuned, you could pretty much get a porting job on a stock shee that'll give you all the time HP with no tuning required, other than jetting.
Now, if you're talking full blown 18 or 20mm Cheatah or Twister on alky, drag quad with all the bells and whistles and you're looking for a few tenths of a second, then...N2O is a viable option...
Two strokes are finicky by nature, if you decide to unleash the bottle, make sure the rest of the motor is up to snuff. O Ringed head, welded crank, etc.