I just put a Athena 440 Big Bore kit in my z400, both hot cams, dirt digger clutch,and it runs great! It has alot more torque but I think i'm going to change sprockets to get alittle more MPH out of it though.

 

it took me less than 6 months to 440 mine and to me it was worth every cent I spent on it

 

I have two Z's with big bore motors. One is a fully ported Athena434 with the combustion chamber opened up, Hotcams, 40 FCR carb, Dyna ignition turned up by Sparks, ESR Intake, White Bros. Carbon Pro exhaust, and DRZ base gasket.
My other one is a Yoshimura450 with a stroked crank, Hotcams, 41 FCR, Dyna ignition, ESR Intake, and White Bros. E Series full exhaust. The head is not ported on my Yoshi. My Athena revs better and is faster on the hill, but my Yoshi gets out of the hole quicker. I will be pulling the head off and getting the same head work done as my Athena.
Both of my Z's have no trouble beeting piped, filtered, and cam modded YFZ's. In fact my Athena has beaten YFZ's with some internal motor work.
The big bore kits are only as good as the builder. There's plenty of power waiting to be uncorked in the heads on the Z's. Just bolting a 440 kit on won't get you the full potential of the kit. You need everything to make it scream.

 

Yea about the 453 I think you're talking abuot the CT kit.

 

if i were you i would just go for something like a 434 so at least the if you had some kind of weird problem and blew you motor then you could bore it out again. Like some people will be going down a hill hit a bump (small or large doesnt matter) have their hand slip off the clutch and be going just fast enough to blow that sucker up.