It seems reasonable to me, although I don't really like the size booster, because effects that 'increase damage as though it were X sizes larger' tends to stack with others like them. It could get REALLY ugly with a greatclub and a friendly mage's greater mighty wallop.
d10>2d8>3d8>4d8>6d8>8d8 without even really trying. If you made it "heavy" it would shoot up to 12d8 per swing, on top of all the bonus damage he'd be dishing out from a likely maxed strength score, power attack, et all.
Although it does make for an amusing merciless greater vital strike Mighty Blow lol. (36d8+str bonus*2.5)*2 damage as a standard action. Good luck getting that past a GM lol.
The one good thing about how it functions with Pathfinder, is that the size changes are minor. Each time it gets treated as a size larger is just a plus 1, otherwise it would be getting a massive boost. (Also, you would need to houserule the past collosal part to just +1 for each effective size past collosal.) Additionally, I would suggest introducing a clause where creatures his own size category are still capable of grappling with him, even if his 'effective size' is larger than they normally could.
That permanent mindblank is very good, and perhaps belongs at level 20 as the capstone considering that virtually eliminates their big weakness (shizzy will save) but its not that imbalancing considering wizards have been casting it since level 15.
