dunno if you ever got any dyno testing done mike but i totally forgot way back when, i modded my intake horn based off your thread and did some runs before and after. granted there was some time between me making the runs, conditions were better (cooler weather with the modded horn), i had lighter tires on with the modded horn (only by 5lbs each), and it was done on an inertia dyno with 2 3k rollers. Same tuning used through out, no changes other than the horn modd and what i stated above. i was on my race tune on both runs (3100uS). Before the horn modd, i put down 465hp. after the modd i hit 480hp. the horn was cut down via a carbide bit, die grinder, 3" round flapper wheel and i polished the whole inside while i was at it. i attribute the 15 hp gain partly due to nicer weather (first run with stock horn was 105* out, second with modded horn was 85*). So i know its not in the best situations but i think its definitly good for a few HP if anything
I did notice quicker spool up off the line when i lived in Flag (where i first did this). its 7k ft in the air there. Down here in phoenix it really shinned. it definitly added a little more whistle to the turbo.