i have a stock pipe on my 88 race head motor, but i have a custom tip similar to the commercial ones out there and the tip did wonders for the power... i made it on my lathe when i was bored one day
my friend has the exact same motor as me and bought a cheapo flyin'50's pipe for 79 bucks on ebay and it made a hell of a difference in the mid range top end, but sucked a little bottom end out of it...the thing is literally a monster on the top compared to mine now ( but he also has the new race head with teh bigger valves)