To answer your questions on much HP can be gained on tuning I've seen 60 HP gain on a pump gas motor from the first pull to the last pull 34 puls later. That involved some cam timing, ignition timing, different range spark plug heat ranges and carb. tuning.
I've also seen only 20 HP gain from the same effort, I'm thinking the differences between those two motors was short block preparation and a good set up on the carbs to start with
No two motors prepared the same will like exactly the same thing on all the tuneable components