What power is a standard unmodified air box with a K&N filter element good for ?

Personally over the years & trying different set ups, the easy breath filter in the standard box is the most efficient.

You're only going to see a more responsive engine with this.Certainly no big gains.

Just to re-phase my question what I'd like to know is at what power does the original air box become restrictive and need replacing with one allowing more flow

i did an OEM carbon fiber copy with improved internals
the gains are not huge but the car is more responsive
and the sound amazing

The air box plus K&N would be well up to the job. There would be other things you'd be changing before reconsidering this. As said you could rearrange the internals for a bit of help, but in all seriousness you're talking about very large horses before rethinking the lungs.

as soon as you start running a bigger turbo it'll be a restriction, but then so will a stock exhaust etc..... you'll only ever be as good as the most restrictive link.

The airbox is easy to change, and does not cost much, so it is usually done early on in the modding process.

On a stock car going to a cone filter will get you a little more top end - but you loose low down torque, with a bigger turbo you are always going to get more top end.

Thing is, on a drivable road car most want more lower end torque. Going down the cone route can also give the placebo effect a nudge with the sound track . Then again it depends what you want from your vehicle.

As I said earlier the air box would be the least of your worries so to speak, other things would require seeing to.

The air inlet hole on the front of the airbox is the same size as the air hoses that go to the turbo so you won't lose or gain anything from that.

For every 90' bend in an air inlet pipe work route you will suffer losses so the airbox adds a few of those.

Airboxes are great for low down torque but probably become more restrictive at high revs.

What sort of driving do you do?

The car is an everyday driver with track day use. I am hoping it will have in the region of 350bhp. I'm not a big fan of cone filters and would like to keep the standard airbox unles it has a detrimental effect to performance.

I'd stick with the the airbox, it sounds less chavy and goesnt really have any detrimental impact on performance.

for 350bhp you are going to need a front mount Intercooler - in which case you can get rid of a lot of bends using a straight induction pipe, which will need a cone filter....you could get there on the airbox, but...

A front mount intercooler introduces a whole load of new curves Si! I'm guessing more than a standard induction route?

yes it does, but you can shorten the induction route when the stock IC is out of the way...which helps compared to an FMIC and stock induction route.