A stock filter has a cone inside and the material is different = restricted.
Also it’s a bit smaller than K&N –
K&N can push 3x more air = cotton filter in oil coating – so cleaner bike & more free revving yes. “650 specific” K&N is good otherwise , BMC Italy does it too (known for race filters on aprillias) – I owned some BMCs in some of my bikes , some K&Ns – they do the justice
However – an upjet is mandatory (hence carb tutorial on the forum goes in better detail on jet swapping – just search “carb” at the top of site.
Running 3x more air on the same stock jets = engine is much hotter , misfires , new piston rings soon lol.
Likewise, if you have a performance exhaust pipe –> more gases going out –> upjet is mandatory
Any air upgrades basically (intake or exhaust) = upjet
// Meditation doesn't mean you have to sit still....
K&N filters are cleanable and have a 1 million mile warranty. The 650 one was £54 and the standard one was £34 which needs replacing every service. It’s well worth buying a K&N as you won’t need to buy another one ever. The fact it adds a little power and helps with fuelling is a bonus.