so I take it you haven't had one ? yet can offer an opinion
I bought a set off H Dev purely because I liked the look (worked for me in terms of colour effect!), it was cheap at 29 quid and under the slim hope it might make a noticeable difference, despite trying hard to feel the effect, there was none.
You can see the charts from H Dev here http://www.h-dev.co.uk/product_info.php?cPath=78_169_293_472_477&products_id=25417
But having been on the VX forum and the Skyline one and this issue come up, almost exclusively everyone who fitted these said they felt no change in the drive of the car.
Bought mine cheap as someone on the forum was making them and it immediately made the idle smoother, you could tell by the sound and needle, obviously poorly earthed at the factory , only had 16k on the clock and not quite 2yrs old so definitely not dirty connections
I did like the look of it though
This is similar to the uprev, even though the improvements can be quantified on the dyno, the opinions vary massively from its a must have to it made no difference ?? And at £400+ that isn't good