There will be no system link.
I'm not sure what you would consider a proper answer, but there are a myriad of reasons why it isn't in there. For one, the networking protocol is peer-to-peer. So once you've authenticated with Xbox Live/PSN and started a game, the lag from one machine to should be negligible (avg broadband speed in the US is 5Mbps). Thus, the only real place where a system link scenario is relevant is at a LAN party with no broadband internet connection between machines. I personally haven't been to one of those since the late 90s.
As for net connection differences - we test across a variety of scenarios. We test globally (US-EUR-JAP games), we test two territory, we test locally, we test on a variety of net connections/speeds (I think we have like 10 different service provides running lines into the office, or something silly like that), we test with latency simulators. We've banged on this game as hard as we could in our testing environment, and it has help up pretty well. Obviously, the global games can experience some lag, but that has more to do with the speed of light/distance and hardware latency that it does with the netcode.
The feedback on local multiplayer is helpful though, as your feedback will guide any future multiplayer experiences (nothing to announce!) that we create. I've been sharing it with Inaba-san.