I think programming is part of it but if I recall correctly there's also some components which are 'beefier' when the engine is uprated to higher horsepower. The ISC 8.3L was capable of being tuned up to 400hp but if you actually needed that much output there were a lot of other considerations and probably cost wasn't one of them. Contrasting school buses and RV motorcoaches, school districts opting for the 8.3L were already stepping up in price over the much more prolific 5.9L and no doubt had some justifiable reason for the premium engine option in that bus. Nevertheless, they still aren't likely to make the leap to an RV rated hp 8.3L simply because they're not expecting the same kind of performance from a school bus that an RV owner who just dropped north of $500k is going to expect and that's the big difference. So if you're okay with 0-60mph in 30 seconds instead of 28 seconds and you don't plan on towing a 35 foot enclosed trailer with a racecar and full pit garage, I am sure you can find plenty of other things to spend your money on than squeezing 30 more horses from your engine.
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