You are absolutely right about the air filter and the flow through the intake manifold. The main reason I don't really talk much about it (I guess I should so we'll talk about it now) is because the air filters and the style and shape of the intake manifolds are for the most part pretty straight forward. You will less likely have problems with airflow to the engine than selecting the wrong pipe and /or having the wrong tune. The airfilter as you mentioned can certainly affect an engines perfromance, not so much on these engines with the length and shape of the manifiold but more with the filter density and oil thickness in the filter. I have seen 280hp 1000cc pro stock drag engines loss over 80hp because the oil on the K&N filters was too thick and the engine couldn't breath. The same could happen in these engines so you are definately right about bringing that up and it is a very good point to make to use a good filter and filter oil in order to maximize airflow but also keep the foreign debris out. I myself use the mugen round filter that came stock on the MBX5 series cars and it may come on the 6 series but I don't know since I don't run them. The main reasons are 1) good density foam but very breathable and 2) $9 for 4 filters instead of $4 per filter through xray. I also use PJ1 filter cleaner and filter treatment on them (the same treatment used on dirtbike air filters). It works very well as there is no signs of dirt entering into the engine through the filter. The inside of the filter is perfectly clean even when the outside is completely filthy. I usually change my filter several times a race weekend to keep the engine breathing properly and running strong. Even though the dirt and debris stay out of the engine doesn't mean the filter is getting clogged and not allowing the engine to breath properly. If you are good about keeping the filter changed out on a regular basis, use a good filter and filter treatment, the engine will perform just fine in most situations. I know K&N makes a small filter that will fit the carbs we use but I wouldn't trust the filter in offroad conditions. I had a couple of them in the past and while they breath great, they also let alot of smaller particals through causing premature engine wear and sometimes catostrophic failures. Companies like Losi sell a 4 pack of pre treated filters for like $6 and alot of people seem very happy with their overall performance and engine protection against foreign debris. I haven't run them myself but from a standpoint, they look like a good buy for the money.