To clarify a bit:
The max stat weapon is a +9 total to your CHA STAT. +6 CHA at Tier 1 (will not stack). The +3 CHA from Tiers 2 & 3 does stack.
Another way to build the stat weapon is to pick a Tier 1 benefit that you like, then for Tiers 2&3, add +3 exceptional CHA (which will stack with your +6 CHA item you are wearing).
The skills item is a +6 total (ALL OF IT STACKABLE) to your skills.