Depends, if you have a de-cat pipe then yes you will loose a bit in the bottom end, but mid range will be improved. The K&N57i is a good filter, I have one on my V6 but I have made my own sealed unit with a cold feed from the bonnet. They don't work aswell as the expensive one as they are usually designed to be sealed units and like the twister fit more cold air into a space than the normal 57i type filters.
Getting the exhaust pressure is also an essential as when I first put the filter on mine, I had no sealed box or cold feed and only had the back box of the exhaust at that time and my god did it die at 3000revs if you floored. You could almost hear the engine choking to death lol. All because the engine was suckking in too much air than what it could get rid of. So hense a grand to gas flow in the inlet, inlet devider and throttle body, replacing all rough, concertiened rubber and then another £800 on the exhaust just to get the pressure perfect through the engine.
Quick shot to give you an idea of the sealed 57i kit:
Best part, not including the samco hoses and flowed inlet piping, the sealed unit and filter plus a few Do It All fixings lol costs less than £120. Which is cheaper and does the same thing as the Pipercross Viper kit and they start from £150 iirc.
This good or rubbish? 
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How much difference would a k&n k57i induction kit make compared to the typhoon (as the price difference is a hell of a lot from what i can see)? Also, would the exhaust i have be good enough to mean that i wouldn't lose bottom end power if i fit an induction kit?
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