Ported heads and bigger valves

Obviously this is Duramax related question and I have a thread started in the DD forum but I'd say there is probably a ton more Cummins guys that have higher flowing heads that might be able or willing to weigh in on the subject here. What gains/benefits have people seen running ported heads and bigger valves? Either factory ported heads like socal stage 1 and stage 2, eldelbrock, Wagler, ect... The question is more based toward a big single turbo truck for mostly competition. Do they seem to help spool up? Has anyone done back to back dyno testing with the heads being the only change? What about running the largest valves in the stock bore vs. going say .020 over or .040 over and getting the biggest valves. I probably have more questions but this may get a conversation started. Thanks.