I actually think the Save and Save and Exit was created as a fault in the bloglib not being able to do a preview.
The idea behind save in blogs was for importing or uploading a picture.
Hack away. All I guess you would need to change is the location in tiki-editpage.php that redirects to the tiki-index.php of the edited page.
Why not join the Tiki Team with a CVS account on SourceForge and commit your modification into our CVS directly