A reed spacer, if i'm not mistaken, will allow more volume in the engine. It can help the mid range, but i dont think it'll do much good for a stock scoot..(maybe after some light mods)...
The boost bottle captures the extra "air" and stores it so it's not lost out the exhaust. It keeps a fresh "charge" of air to help "boost" the low, mid range. Of course it's not going to be a drastic change or anything, but ppl swear by them to help. In the high end race set ups is where they say they dont notice a difference......... OF course their is always the debate of weather they work.. The only consensus is they DONT work on a 4t.....(i have one for the ZX i'll be trying myself) ...
I would wait on the irridium plug untill after the carb,etc. is tuned in...A $12.00 plug chop each time will get expensive QUICK....(then i think it's a 9 you'll want)