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Guys I know with cutouts have said not much to a tenth in the 1/4. I plan on putting in my FLP cutouts with McCord electrics in a couple of weeks - as soon as I can get the Stingers off and the Ti's in.
What I heard was a couple got no measurable increase and one guy said he improved his quarter by a tenth. Take into account that these installs were on totally different setups. A couple I saw were on esentially stockers and a couple were on highly modded cars.
I also saw a cutout on a C5R-blocked 427, 600 fwhp Camaro about 8 months ago. It sounded downright badazz. The cutout was already there when the tuner got the car (it was fresh-new from the dealer) so I can't say what/if it added to the output the tuner quoted.