Pods are OK in the sand, for drags, ice racing, road racing and maybe a TT if the track is watered well, but for anything actually dirty they're no good. I've never done that to a 250R, but on RZ350s (a streetbike with basically a Banshee motor) when you switch to pods (or a Y-boot like mine has) you take out your 230 mainjets and put in 420s! It's possible to nail the tune, but to do it you gotta mess with a bunch of needles and clip positions and fiddle with the airscrew all the time. A 250R is probably a little more forgiving, but not much. On the RZ, the huge mainjet kills your gas mileage, down 10-mpg at least with no extra power whatsoever; guys have tried and the RZ airbox is much more closed than a 250R one with no lid.
I'm sure airboxes come up for sale here quite often as people put on aluminum ones and do whatever else.