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Phil, Yes you need a front mounted UV/IR filter.
You can see on the attached graphs for the 350D, the filter glass cuts off Ha (6500) and SII (6720) severly, but the filter removal, despite improved Ha, SII response, has nasty response bumps after 7000. The front filter (I dont have a graph) smooths these out to give natural response with an extended red curve. I dont know if the standard in cam white balance is still valid, but it would be close.
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