I think a better direction for this build would be to target 2006 and newer models. The reasons being : 1. All the current headers are for 2005 and older N/A Subaru 2. The larger demand now is for a true BOLT-ON solution to 2006 and newer owners. 3. This way you would no be entering into a saturated market for EL or UEL header for N/A Subarus
If you only target 2006 and newer I would build UEL primarily and EL in lesser numbers because 1. The stock EL header is VERY efficient so much so that most EL aftermarket Turbo headers are based off of the Subaru design 2. IF you venture across the various Subaru forums you will sense a stronger desire for the bolt-on UEL option.
The majority of the guys wanting aftermarket headers aren't running built setups so you don't need to spec the header primaries too much larger to run more or equally efficient to the stock unit. For the few that have bolt-ons the maximum extent would be cams in which case the stock header still would flow sufficient.
Many guys are going to say "well I will not pay that much for sound or just for the burble" blah blah.....
What it boils down to at the end of the day is that you make a cost effective UEL header that is 100 percent bolt-on solution for the 2006 + crowd. Yes that includes a solution to the integrated cat. I would suggest that you build the UEL header with the cat integrated in the stock location that bolts to the stock mid pipe or make a Header mid-pipe combo and included O2 wire extensions in the kit.
In the end you will probably make a stand alone header and a header mid-pipe combo that will address the cat and stock sensor location issue. If you can integrate a converter in the header that utilizes the stock sensor locations and keep the price down youll sell a bunch of them. If the converter means higher price consider leaving it out altogether.