I ran a 383sbc with 12.5:1 comperssion, solid lifter .595 lift cam, fully ported 461 feulie heads, and a gear drive in a 69 Firebird. The guy who built the engine was an old school speed shop owner who wanted to build an engine with vintage tech and a vintage feel. Sure it is outdated tech, but it ran 115mph in the 1/4 in a 3500lb car on regular street tires with ease. The gear drive made itself known for sure. For the times I took it to the track, I got a lot of strange looks because of that noise, probably out of confusion at to what was causing the blower like noise under the unmodified factory hood. Personally, I'm certain that it did absolutely nothing for increasing performance, just attitude. Frankly I was happy to send the complete motor/trans on to a friend to replace it with a bit milder(and quieter) 10:1 comp 406sbc with a double roller.
Reliability wise, Smokey Yunick swore by those gear drives for years in his race engines! They couldn't be all that bad...

George