Hi Jeff,
I maintain a small group of school buses with the smaller MBE906 as opposed to your MBE926. The engine does have a pretty bad turbo lag, however a lot of lag also comes with the throttle ramping rate which is computer settable. Especially hard hit are the california versions which hang the throttle on the turbo boost VERY badly. Your local DDA dealer can change the ramping rate to give you a bit more get up and go.
I hope that this engine works out for you, I've had many issues with them. I've had air compressors blow up, oil pumps snap shafts for no reason, and every engine i have is already on its second or third water pump, in just 3 years of operation. Granted, under warranty, but puts you out of service for a few days while they get around to doing anything. .
Oh and just some tips on the engine. DO get a couple extra injector crossover lines (from the unit-pump to the head). They rust from the inside out VERY quickly. I've had to replace just about every one of the small hook shaped lines in just 3 years of useage in our buses and it seems to be unrelated to fuel quality... They seem to be made of poor steel and are poorly flared on the ends which leak very easily and will ruin your day if it leaks fuel all over the place (they take literally 5 mins to replace). DO get your filters from FRAM... CH9260(oil cartridge) and C9262(secondary fuel). They are much cheaper than the mercedes branded ones and not subject to the mercedes surcharge (i think they charge for the hologram on the box. )
Good luck with your new truck
-Daniel