With the addition of the urea system on the 2010 EPA engines, Cummins is rating the ISL at 450 hp. This also required Allison to bump up their max rating on the 3000 series as that is right at the top of its range.
From my understanding that with all versions of the ISL the torque has been the same even though the HP has gone from 370 to 400 to 425 and now to 450. The Allison MD3000 is rated to 1250 ft/lbs. of torque so I guess they figure there is no need to go to the more expensive MD4000 transmission as long as they stay under the max torque rating of the trans.