Modifications? We don't do any of that malarkey round here....
A pop charger is basically an short intake induction kit/air filter. Doesn't really offer much (if any) performance improvement, but does give you more noise If you want to achieve a very similar result, but for no cost, do the 'Wakbox' mod that I'm sure you're familiar with I did this when I first got my Zed, but eventually got a pop charger anyway
If you want to see one, I just added a pic of my engine bay to p7 of my zed thread
Plenum spacers I don't really know too much about (apart from what they are and that Wasso sells 'em). They don't offer much in the way of gains unless combined with a remap etc I'm led to believe.