i personally haven't had any experience with the e-bay downpipes, but as i look at them
http://cgi.ebay.com/ebaymotors/ws/eB...spagename=WDVW i can tell that this downpipe will replace the cat. that is, you will have to remove the cat for installation, and so if/when you get your car inspected for emissions each year (granted your county tests for emissions), then you probably won't pass emissions.
so this is still viable, but for that one day out of the year you would have to replace your stock DP + cat so you could pass emissions.
another possible downside to these downpipes is that the fit might not be perfect, and so you'll have to do some drilling/boring and modification to fit.
ask on 3si.org if anyone over there has tried these downpipes.