lazyjones, Here is a link of a step by step process of speaker removal of a SC430. Post #2 show the speaker specs., looks like you will be able to use aftermarket speakers.
http://clublexus.com/forums/showthread.php?t=288367
Thanks grnsc40095, I clicked on that link and it is a really helpful and easy to follow guide on exactly how to remove the panel to replace those speakers. Thanks!