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If the paper is literally being "sucked" to the tailpipe you have too much back pressure. Back pressure really is only measured at idle or 1000 rpm, or some set idle speed for diagnostics, so in teory there is no "measurement" for back pressure during decel. but it should never be able to actually suck anything back into it from the tailpipe.
Two things to check:
1- Are these aftermarket hi-flow cats. that you had installed, or GM replacements? The cat' is such a dense material that any backpressure beyond them should be almost nil as it is "absorbed" by the air flow changes in the cat and muffler. (Even with straight pipes I've never seen anything suck in like that.
2- Check your timing, and set properly. If you are getting that much back pressure, it sounds like your timing might be too advanced. (Have you by chance changed the lifters, rockers, or the cam?)
Hope this helps.
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