"Oh yea.....they are heavy. " .. LOL...
I drove a 99 Freightliner with a Detroit series 60 430 hp in a swift truck with an Eaton 8. We were governed at 60mph, but up hill with a full load 80,000 lbs we usually passed people. And we got 6 to 8mpg with a full load. My last tip with them I ignored the 60mph limit and coasted down hills from Huston to Denver and got an average of 10 mpg. I real liked those engines. And if you can get the computer with opt idol, it is nice in cold places. The engine would start once it got down to freezing for 10 mins then shut off. So no cold starts.
I drove come cats after that, which worked fine, but they seemed to be louder at speed. More rattle and banging in the same style Freightliner .
As far as the 250 in them at altitude. I still plan to put a turbo on mine. No increase of fuel, just get the air pressure back in the intake when at 10k feet. It seemed to do good in Baton Rouge at sea level, but now in Cripple Creek, it is a dog..