Yes, you can absolutely use a cat-back exhaust from the SR on a KA.
The shifters, like 240sig said, are also interchangable between the SR and KA.
"I also heard that you can put a SR20DET downpipe on the KA's" huh? <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/eh.gif" border="0" valign="absmiddle" alt=':eh:'> That won't work.
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