You would be the guy to ask. The new 11L Paccar they're offering now. Is that a totally new engine or is that something that's been offered over there for a while and just new here.
Personally I think they're going the wrong way. The 12L ISX isn't a big seller in the class 8 trucks so I can't imagine there is a huge demand for a slightly smaller version of the Paccar. Especially how underpowered the MX13 is from the factory. Missing the 15L market which most trucks even the fleets are buying.
I think it's like the Cat C10, exact same as C12 except for crank and cyl bore. So your gaining no weight savings. Seems pointless.