Cool idea on cutting the lid down. Doing the math on just the filter dimensions alone, that 12x5.5 filter would have a surface area of 207 sq in. The popular 14x3 filter would have a surface area of 132 sq in. Looks like a good way to get increased air flow on the cheap.
One interesting thing to note, these trucks come with a 12x3.5 filter stock, which has a dimensional surface area of 132 sq in...the same as the 14x3 filters everyone replaces them with!
Now of course the cotton gauze K&N style filters are going to flow more which is why a reusable 14x3 will still feel like an upgrade over stock...but just some food for thought when you choose your filter. Run the tallest you can that will still let the hood close!