You could always get a secondhand cat from a car that has over 30% more power than you own and fit that and leave it there. Could also fabricate the pipe work in and out of the cat so that it does not affect flow so much. 7 degree flange in, 12 degree's out, and the out pipe 15 to 20 % larger cross sectional area tha the pipe entering the cat. Could also move the cat back a few mm, at least 150 mm. Or is that more than you wanted to know, sometimes I can rant a bit.
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