[mwebb]

not wrong but possibly incomplete

because
maybe
turbulence will be introduced beyond the throttle body
this turbulence May / Might / could possibly result in UN even and IN consistent flow of air to individual cylinders
because the intake manifold consists of a central chamber with mostly equal length and volume runners radiating from it to each individual cylinder

unknown turbulence introduced near the inlet of the central chamber
should / will / might change air flow per cylinder

which would result in individual cylinders being fed the same amount of fuel and varying amounts of air

which will reduce overall efficiency because
the 02 sensors correct fuel trim per bank of 3 cylinders not for individual cylinders

this assumes that the gouge carved into the throttle body has any effect at all - which at this point
remains to be seen and would be subject to change based on the size / shape / installation / location of the gouge

and to the OP
it would appear if what you state is correct that you are not actually a shyster
just an UNsuspecting victim a naive mortal
as you could have purchased a dremel tool with some 1/8" die grinder bits
and created your own gouge , for the purposes of solving your own curiosity for considerably less than what you have
been hoodwinked out of
after watching the many free videos on the net showing how to install the gouge

if what is above is true , a thousand apologies for questioning your integrity

there is however another possibility ...