On a stocko rebuild with normal pistons 170-180rwkw should be fine @ around 15psi should make that kind of power with a decent turbo. If your after more than this its probably wise to at least run forged pistons. This alone will not make more power but will allow you to run more boost.
It is possible to make huge power from a Ca18 ive heard of over 500hp reliably it really depnds on how much power you want and how you want it to make the power. Throw a huge turbo and lots of boost at it should make over 400hp.
My ca18 makes decent power not as much as a lot of cars out there but with a lot more torque and midrange it would definatly be a faster car especially around a race track.
To build a decent forged internal ca18 with roller bearing turbo and cams ect expect to spend upwards or $6k it woudl not be hard to spend $12-$15k with major headwork custom bits and pieces.
Cheers Andrew