Dear Mimoh,
Greetings !!
As you are aware that Air Filter role is
a) Capture Dirt Paricles, Contaminants to get to engine
b) Also controls the the amount of air flowing into car engine. Been Air and Fuel mix will impact combustion in car engine and thus power is generated.
K&N Air Filter what it essentially does is that it increases the air flow mixture to such an extent that combustion properties in engine enhances to extent.
Coming to K&N Air Filter
K&N is a global approved brand name and used worldover by car enthusiasts who seek a pickup boost on overtaking
If you ask in number terms - dont expect significant boost - its marginal but definite observed on overtaking as higher air flow enters in the engine. If you considering performance boost - replace both
> Air Filter with K&N
> Spark Plug with Iridium type - which can provide more spark to the firing tip, adding higher fuel and thus gaiun
In terms of downside - Your car exhaust will feel slight more noisier and Possibility on medium term could be what impact to Air flow sensor and worst case to Engine too and same wont be covered under warranty if same specified due to K&N - so very essential to buy genuine and right Air Filter for need. Also for performance boost having Iridum Spark Plug (which last upto 1.5 Lakh Kms or 10 yrs) can be winning edge.
Rest no changes needed to engine tuning or so for using above filter or Spark Plug
Valvetronic exhaust
This is modification to the exhaust and first of all your car emission and mechanical warranty will be void the time you do this modification
Secondly your car fuel efficiency as end mileage will drop by near 15% to 20% as around 1.5 to 2 Kmpl in real world conditions
Third by spending 30K to 35K or even more for whole set up or even more you will get increased noise from exhaust and though there is a choice of adding resonator to reduce that high pitched irritating bike kind noise. but still noise levels will be there on revving car engine (some calls it sporty and to some its still irritating - you have to see your acceptability levels to it),
Last comes to fitter too - as you need an expert who has expertise in fitting and modifying exhaust.
Rest yes will gain on performance - but do note that if your car noise levels will increase in due to exhaust such modification if caught by traffic police (with exhaust mode on) can lead to challan due to increased noise pollution.