I'm pretty sure the stock air box with a good panel filter will flow enough for 300+ bhp?
I would spend money on exhaust and TIP. Even if you wanted more power than thus would yield your looking at getting forged rods to be safe
I know a lot of people with standard air box/panel filters and say there sound...as I already have a k&n cone filter I thought I'd just buy a whole new induction kit either piper cross or green filters....now I've been enlightened on the TIP I'm looking into that more. I was just going to buy a blue silicone one from eBay presuming they all were the same :S