Ltz400 vs yzf450 whats the big deal?
Well, you answer your own question.
Motor size means something, but it doesn't come close to meaning everything.
Take that Mustang - a Peterbuilt semi has a much larger engine, right? Which is faster in the quarter mile?
The 450 is built for speed. Everything from the 5-valve head to the tiny hocky puck of a piston to the crankshaft that weighs less than HALF that of an LTZ - all comes together in a lightning quick package.
The downside is reliability and rideability.
You can have light weight, you can have big power, you can have reliability. You can't have all three in the same engine (yet!).
Your z400 is reliable, it has good power, it is not heavy, but it's not light either.
The 450's are lightweight and have tons of power. They require rebuilds every season, or sooner or later the engine will have a catastrophic failure.
It's all up to choices - for most people the z400 is probably the best possible choice for their riding style and terrain, but darned that 450 power is just so much fun!
Someday they'll make an engine that's got the best of both these worlds - for now we've got to make choices. Or we could pony up and buy the KTM - it's got it all!