My Volvo is putting 1850 torque out.In an 05 or 06 ? Or in an older. Also how did you know they were Cats. Also depends on rears. Both are good motors but I can promise you a 12.7 doesn't have a prayer against a C15 and a 14l doesn't have much of a chanice either. We're those C12s or C13s you walked past ? Also what's the biggest 12.7 470 right ? 1550 or 1650 torque then it's 515 for the 14l is it more. A 3406E or C15 are very common in 500hp 1850lbfts and can be made into 600 hp's very easily.
I've got a second Detroit with a 12.7 in it putting 565/1850 out of it. Again, tuned by Leon and using a stock engine. No special parts at all.
You think cats are the only ones capable of that?