Some apples & oranges comparisons accidentally going on
!
A Mountain Aire with an ISM (450HP in the case of a 2007 model), is not the same as a Mountain Aire with an ISL (400HP in the OP's 2005 mentioned model).
It always comes down to power to weight ratios, on whether or not an engine is going to support a specific coach.
As pointed out in other threads over the years. Bigger Block sized engines will weight more then Mid Block sized engines. They also get the heavier Allison 4000 vs the 3000 series transmission. Usually have heavier drive shafts, axles and larger capacity cooling systems too. So much of the extra HP/Torque of the Big Block engine is used to support the overhead of this extra weight... No question the ISM450 has more HP/Torque then the older version ISL400. Newmar spec'd the ISM450 vs ISM500, to separate it a step lower down the food chain from say the Essex, and up a bit higher in the food chain then say the Dutch Start (Which if I remember, has a 450HP ISL available in 2007(?))... Normal marketing. Some owners with ISM450's, ave been able to get the ECM's reprogrammed by Cummins to deliver the 500HP power, and I suspect also have improved HP/Torque curves too, over the ISM450.
Other factors to remember when say comparing 40' coaches (And I know the OP is asking about a 43' coach. But many of the examples have been talking about 40' units.) are: A 40' coach with tags, is going to have more weight to carry then a 40' coach without tag; A coach with 0 - 2 slides, will have less weight added from the slide mechanisms, then a coach with 3 - 4 slides; A coach with a 135+ gallon fuel tank, is going to have more weight when full the a coach with say a 100 gallon fuel tank; 40' coaches come with different chassis available. Higher end chassis are usually heavier then entry level chassis Rear vs Side Radiators can also make a difference in weight, as well as power consumed for cooling... (I'll stop here, but etc. on other attributes to coaches...).
All of these factors, makes sharing and comparing how say an ISL400 will perform in a coach, relatively meaningless, without the specifics of the model, year, chassis, etc., etc. of the coaches being shared as examples of how well the engine performs in a coach...
OP - Best of luck to you on your decisions, and if needed, on going hunt for a coach.
And best to all, travel safe,
Smitty