AK - I ran my cat all last winter without my racepak, and didn't burn down once. the racepak is nice, because once you find the baseline temp that the sled likes to run at, you just try keep it close to that, and if the temps do rise, atleast you know and have a warning.
K&N's are nice also because like Taxicat says, they make em in a variety of sizes, and from what i have seen KickA$$ doesn't make their filters with different flanges on em. and on cat triples you need atleast a 20 degree bend in then so that is clears the jackshaft. i went with 4X5 filters with 20 degree bends on my 2000 cat, they fit nice, and once you do get the airbox out of there you can take that little plastic intake out from behind them and it gives you way more room to work.
when on the trail, and using K&N's just check the plugs once in awhile, there is no need to run the sled light brown on the trail, so stay with a nice choco brown and you won't have any probs. make sure to check the mid-range jetting also, and when jetting jet high and go down, running rich doesn't hurt as like running lean does.