The guys who have this really figured out are the ones who run in FAST class. They have to run with stock appearing engines and full stock appearing exhaust. I know some of the tricks that they use but I don't know them all since I haven't done any of that stuff for years. But there is a fairly firm limit on how much duration you can run on the exhaust side when you have manifolds and a full exhaust system. Too much duration and you lose power everywhere. They compensate by running a lot of lift and a ton of compression but they don't run a huge amount of duration. I wouldn't recommend a FAST cam for street use, just bringing it up as an example that you don't want to get too excited with exhaust duration if you are using a full exhaust system.
If you want to see some crazy split cams take a look at what the Chevy engineers are using in the LS engines. The COPO 427 engines have 40 degrees more duration on the exhaust than the intake. Something like 230/270 for duration. I'm not sure how they came up with that. People always say "think outside the box" but I don't think I would've ever thought that far outside the box! Here is a link to a COPO data sheet:
http://www.jegs.com/p/Chevrolet-Performance/Chevrolet-Performance-427ci-425hp-COPO-Crate-Engine-2012-2015/2804804/10002/-1