these are my opinions having not given it much thought and maybe not explaining it the best
tuned systems make greater use of mass inertia with regards to the intake and exhaust tracts. you close the intake valve ABDC to take advantage of the inertia that the intake charge has, it's keeps filling the cylinder ABDC because it's in motion. on the other side you open the exhaust valve BBDC to use "blowdown" to start the gases moving in the exhaust and establish inertia because the scavenging effect it will have is more beneficial than any additional energy that might be recovered leaving the exhaust valve closed longer. of course there's the overlap where the inertia of the exhaust gasses is used to pull the intake charge through the cylinder for scavenging.
the tuned length involves both the intake and exhaust tracts and has to do with the column inertia and pressure waves.
properly tuned naturally aspirated engines can achieve >100% mass efficiency.
imho with a forced induction engine the exhaust tuning would have a rather negligible impact compared to a NA engine being you have a greater pressure differential across the engine. there would also be a secondary impact of additional heat loss with longer tubes and that's energy lost that could be used to drive the turbine.
imho the main purpose of a tuned exhaust system on a NA engine is for better scavenging, with forced induction you can clear the cylinder much more effectively plus percentage wise any exhaust gases that might/could be left wouldn't dilute the fresh charge as much.
i didn't read any of this but there may be some information of interest
http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm
Even an old fart like me could understand it. The only thing you left out is room constraints, which, when dealing with door slammers, has a major impact on design.