I have the following question:
What happens if the Sion becomes a 6 man troop? (why not a 9 man troop???) => Life, Damage and so on...
6 x Damage?? So a Warrior can heal himself 6x6 per hit?
It's not a 9 unit troop because the game is configured for max 6 unit troop (although I believe this may be moddable.) Note that Butchermen, Assassin demons, etc. are also limited to 6 units per troop.
And yes, you get the same effects for Sion units as for other units - 6x attack, 6x hitpoints, etc. The effect if you give them Reap unit trait is the same as for a 6 unit "normal" troop with Reap.
Literally the only benefit of this over "regular" troops (though it's a doozy of a benefit) is that you can pursue the traits in the various hero paths. So, for instance, you could make a 6-unit troop with the Commander path that gives +XP / + init perks. Or you could go the Warrior path and pursue Lethal perks + your weapon of choice perks. Or whatever.
Basically, it redeems Warrior/Assassin Sions (which are usually outclassed by regular units by the time you can create them) and doesn't do a lot for other Sion uses, other than providing extra durability (albeit extra durability is nothing to sneeze at.)