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K&N in-airbox filter: minor performance improvement at the cost of minor filtration loss at higher RPMs. Some jet tweaking desirable in conjunction with it, but not absolutely necessary *if* you are using the right restrictor donut on the air-take side of the filter (but if you're going to choke the intake to make up for stock, why bother changing filters to a less-effective filter in the first place?) and stock-sized exhaust header solutions. K&N dual-Pod filters (1 filter covers 2 carbs, 2 filters): intended only use in conjunction with aftermarket headers (complete aftermarket exhaust systems) PLUS a full rejet. This is intended primarily for people who race their bikes (drag, etc.) and who value extra power in specific RPM ranges (typically street racing) over smooth power through-out the entire rev-range (typical street rider use). Nightmares getting these jetted correctly across the complete RPM range abound and many KR members have reported going back to stock after trying to get these jetted perfectly without success. Results vary with different brands of exhaust headers because of differences in designs. K&N single-Pod filters (1 per carb, 4 filters): intended only use in conjunction with aftermarket headers AND some form of bore-over [displacement/compression ratio change] kit PLUS a full rejet. This is intended primarily for people who race their bikes (drag, etc.) and who value extra power in specific RPM ranges (typically track racing) over smooth power through-out the entire rev-range (typical street rider use). Like the 2-carb-per-filter pods, nightmares getting these jetted correctly across the complete RPM range abound and many KR members have reported going back to stock after trying to get these jetted perfectly without success. Oddly, these seem to have fewer issues than the other form of pod filters, even without any engine bore-over kit use. K&N in-airbox filter: No performance increase on the dyno over the stock filter. Some jet tweaking desirable in conjunction with it, but not absolutely necessary *if* you are using the right restrictor donut on the air-take side of the filter (but if you're going to choke the intake to make up for stock, why bother changing filters to a less-effective filter in the first place?) and stock-sized exhaust header solutions. Specifically NOT recommended by both Ivan (of Ivan's Performance Products) and Marc (of FactoryPro fame). K&N dual-Pod filters (1 filter covers 2 carbs, 2 filters): mail me $200 to $500+ you'll spend on dyno-time and shoot yourself in the foot now -- it'll be cheaper, easier and less-painful in the long-run. No one has ever reported being able to get these to run correctly (i.e. - permitting fueling of the carbs reliably across the entire RPM range from idle to 10k) on a 98+ stock engine with any form exhaust (aftermarket or OEM). We've had several try for months (in one case a year) on end, including a couple guys who spent literally days on a Dyno & exhaust analyzer with various jetting trying to tweak the carb fueling so that it wouldn't flat-spot in one RPM zone AND foul plugs at some other RPM zone, only to give up and go back to stock. K&N single-Pod filters (1 per carb, 4 filters): pretty much same as the two-carbs-per-filter above, although there have been sporadic reports that people have gotten it to work (note: all the positive 4-pod reports on the 98+ have always been from squids/newbies who didn't have the experience to necessarily know whether it was better than the results they would have gotten without the pods given a good jetkit). 
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