Actually, I was considering using a drop in filter with my ITBs. The inlet and outlet of the stock airbox are both 3 inches. Figured I'd just use some mandrel pipe in between TWM's airbox and the stock airbox, and drop some pipe into the wheelwell. After some research I found that the stock filter has the precise surface area that K&N says is ideal for a 1.8L engine revving to 8000 RPMs.

So I bought a CX K&N, only to discover that an inch of the pleating on either side of the filter is plastic. Returning it tomorrow.

Other than that, it would work fine.