with Inscription you get 1% SDI for every 20 points, then 5% bonus for being at GM. total is 10%. so if you always play a human it's really 9% SDI because you always get 1% from Jack of All Trades
as others have said it does not count against 30% SDI item cap. so you can have 30% SDI from items and 10% SDI from Inscription at the same time.
also just as a side note you do not need Tactics to use Wrestling special moves.
keep in mind there have been changes to Magic Reflection and Magic Resist that you might want to look into before making a decision about Inscription but someone else would have to clarify on those.
edit: I just read in another thread that Casting Focus is capped at 12%, but if you have GM Inscription it is capped at 17%. casting focus gives you a chance to avoid spell disruption, kind of like Parrying or Evasion