No, inside the plenum, where the air flows. There are different schools of thought that a smooth air flow or distrubted airflow are better. I have no idea, I dont know about fluid/aero dynamics so dont know which is better. The only evidence I have seen is that the Leon lads used to smooth their airboxes of the platic veins that they had and it gave better flow. I would just stick with smooth for now as it would mimic OEM?
Why would more air mean less fuel? Surely more air would mean more fuel to keep the AFR the same? Surely the problem you get is that the parameters only allow so much fuel to be delivered (as stock only flows so much air and we are going past that with a bigger plenum), so a remap would force it to deliver more fuel and hence more power. The stock setup will only lean out so far and then wont use the extra air that can be taken from a freer flowing intake. By remapping it, you can use more of the air combined with more fuel to give more power. I'm still not convinced that a much bigger plenum will give any gains anyway, there will be a limit where it makes no difference or will be detrimental. Sticking to a design that is like OEM + spacer will give best guarenteed results for the outlay as its a proven combo