Rich:
ive seen this setup on quite a few aftermarket silencers- it seems like it is a trick, just like the SST setup that has the necked down section at the end of the expansion chamber. ive compared a couple of pipes- on the SST and the fatty, the front and mid sections are the same, but the SST is longer and then it has the thinner section- doenst theory say that the shorter pipe should rev better, but i and others have found that the longer SST pipe is more of a topend pipe than the fatty.
as Bedell suggests, it seems like they are trying to do a similar thing with the silencers- run a shorty and get the improved bottom, but have the reducer in the end to trick the engine into thinking it has a longer unit at higher RPMs.
as for piston temps, that wont be a bad thing since my YZ250 runs real rich- i assume because Eric has ported it and the scavenging is improved, once again, ive run out of needle to lean it out.....i was going to ask you how the scavengin works, but ive managed to convince young Billy to buy the Graham Bell books- i just need to convince him to lend them to me now :)