The support for the GC adapter will have only marginally better latency if any but will 100% accurately represent gc inputs (namely analog stick) although the difference between that and say a mayflash isn't really noticeable at all.
It wont be added to the netplay build because it's old an outdated but when UDP netplay is merged to the master branch (allowing the 8 digit codes to be used, not sure if anyone at all is working on this anymore) we will be switching off of 4.0-652 completely. Dual core Netplay is already in the master branch so you could play melee perfectly fine on the newest builds with native GC adapter support if you use the port forwarding method and your opponent has the same Dolphin revision. My friends and I exclusively use the newer builds.
Most exciting about using the new builds is much better performance, exclusive fullscreen (reduces input lag much more than an adapter will), FOD lag fix, removed 200 ms audio delay, and stuff like that. UDP Netplay is literally the only thing holding back a huge switch to the new builds. Although I think there are some Wii Netplay issues but I'm not sure about that. We'd have to bug comex to add it.