Please explain why different, assuming more flow-optimized, headers do not add to performance?

Say you have a Turbo with the simplest tune - exhaust + chip. I've heard adding better headers would give another 30-50hp in the upper part of rev range?

 

This is a great case of throwing money into a car for negative gains. How can headers decrease performance? Add bigger ones, decreases you flow velocity, slows spooling of the turbos. May even decrease your boost pressure. If you don't compensate with a tune, you're screwed. In the heat, I would definitely focus on tuning, as heat can decrease boost and then you lose power.
You mentioned "major service" coming up. How many miles? 40k? Plugs will definitely affect performance. Always a big debate on plenums with NA cars too. You need to hope it looks pretty, because performance increases are an illusion.