Ive had a Power Boost Valve in the past and there pants. If it doesn't make a noise you haven't set it up correctly, and if it does make as noise it will drive you round the bend and you'll take it out in no time at all. It made no difference to power or MPG. You'll also find all the main tuners will map a z22se from Astra's, Vec B's and the like but not in the vec C as its a different ECU.The slight lag you mention is sort of built in as such, I belive it comes from the fly by wire, its hard for me to notice now but its more noticeable after ive driven a cable operated TB, I have found however that a K&N panel filter fitted in a factory airbox does seam to give a little better response but no more power.
If you take the TB off you'll find the manifold full of oil and gunk from a mix of EGR and cam breather. I blanked my EGR but kept it connected to the loom in preparation for a 2.4 manifold (which I never fitted) and the manifold seams to have cleaned itself up, the inside of the runners look far cleaner.
Unless you want to spent plenty of cash on her then id just keep her serviced well and forget the gimmicks. I have a 53 plate vec C by the way.