My only experience with Brass is my stock Sterling Barrel. If I'm playing at a field (FPO field) and their paint is .688'ish I do notice a difference between my .688 insert SP Teardrop barrel to the stock barrel, I'd much rather use the stock barrel it's more accurate, plus it just sounds cooler!
I don't have any brass inserts at all but once the smaller inserts are available again I'm going to be buying some various sizes to give it a test. :-)
Started with a Splatmaster, moved onto a Nelspot 007, then came the Autocockers, then a STERLING Pump, next was a Bob Long defiant then a PM7. As of now I have 6 Sterling Pumps, 1 Phantom for SC, a CCM SS-25 and a Sovereign 3.