Odd being restrictive, heard this crap before, no doubt started by a competitor exhaust company, or some nob head assuming the smaller diametre at the rear meant that.
When swapped around on a certain 530bhp yb with another competitor exhaust the difference was 1.8bhp, in favour of the scorpion, this difference would be natural in the same run on the same spec, as they do vary so in essence no difference either way, the competitor was a Mongoose.
tabetha