This is not my actual experience and I disagree with your assertion. There are measurable differences between how Conical and Flat Burrs grind and this is widely accepted to affect flavor profile.With any grinder, a ristretto pull from any roast will always taste syrupy and thick. Grinder burr geometry is another snake oil and should be double blind abx tasted
I will agree with your statement insofar as you limit it to "specialty" burrs. Is an SSP $300 Burr set really so different in final taste that it is worth the cost compared to what comes stock in my DF83V or a $70 set of Mazzer Burrs? To that end, this is the only reason I haven't changed out my stock burr set for the 83mm Mazzer Burrs: while there may be some differences, is it the difference that does exist between Flat and Conical? (I'm not so certain, and thus not willing to freely give away hours of my time changing, aligning and seasoning new burrs in a grinder that thus far has failed to perform in a way that satisfies my love for espresso. The DF83V is a good grinder for someone... just not for me.)