Installed my second Ivan's kit a few months back - not too much to fear, particularly with the excellent instructions online from Pat & Mike.
I have however come to a different conclusion than nearly everyone else - I think that if you're not after top-end kicks, Ivan's Slip-On kit is arguably better for legal(ish) road speeds.
Ivans graphs certainly show a considerable torque advantage between 2700-4000 revs for the slip-on kit, possibly more too if the graph curve started lower down the images. True, it loses out to the Full Monty kit with the kick @ 6000-7500 rpm but by then you're already over the limit and entering warp drive speeds
Furthermore, being miserly I always get over 40mpg and didn't have to splash out for an aftermarket filter. Also, she still runs great when I fit the restrictive standard exhaust and I even ported my boots for more optimum mixture flow.
Then again, perhaps because both jet kits are the same price, people nearly always opt for more power. But about 130 ponies with Ivan's Slip-on setup is enough for me