A couple of notes:
The short chainstay has nothing to do with tucking the rear wheel in. With this chainstay length and the corresponding front-center, it allows you to have a good weight distribution between the front and rear wheel (the basic concept is, your body moves forward on a tri bike, so the wheels should shift forward too to keep the balance). On steep descents, the opposite of what you think will happen actually occurs. On a descent, the bike is tilted forward, moving your weight even more forward. So with a long chainstay that can cause a problem, not with a short chainstay.
We can make a P3 with any kind of chainstay length, we chose what we chose because of that weight distribution. Note they are also no shorter than what we used to do on the Eyre Tri seven years ago, which didn't have a rear wheel cutout. It's just what we feel is the right dimension.
To prove that point, you will notice that a 51cm 650c P3 has the exact same chainstay length as a 52cm 700c P3, even though the wheelsize is obviously a lot different. Then when you compare a 52cm 700c P3 to a 61cm 700c P3, you will see they have different chainstay lengths even though the wheelsize is the same. And for all these frames, the cutout spacing is the same.
The same goes for the P2K, even if wheelsizes are different we can create the same chainstay length AND create the same seattube cutout clearance for the rear wheel.
As for stuff getting stuck in the cutout, you control this yourself. If you keep the tire clearance UCI Legal (i.e. you can stick a credit card between seattube and tire), nothing will get stuck. If you tuck the wheel in as deep as you can and leave almost no room, then stuff will get stuck on rough roads. I had this happen in one race when I went off the road into the ditch, and I had my rear wheel set very tight (this was in 2000, we had little experience with the P3). So it took a few revolutions for the cutout to clean out. But if you allow just a little bit of light between tire and seattube and stay on the road, it won't be a problem.
Gerard Vroomen
3T.bike OPEN cycle