I agree with Ray. I ran an undertank filter for quite a while. Although the bike felt fine, the filter was fitted when I bought the bike, before I started selling the MPX038's that fit in the tubes I tried them first and noticed a marked improvement in throttle response and maybe a little more kick up the top end. The induction noise is certainly louder
They are far thicker than the stock filters, and a little bigger so seal correctly in the tubes and seem quite happy to run without the mesh over the front of the tubes to.
To some degree I still suggest the undertank filters on the earlier bikes with the thin tank/airbox seal as this lets dirt in. I've replaced mine with the later one which is a lot thicker though and now enjoy a dust free airbox
The '38's are washable and seem to last pretty well and at £25 a set, cost little more than the stock tube filters but a lot less than the undertank items.