I figure the most influential thing Ace did was being a big pioneer of his self-titled "onbeat/offbeat" style and his rhyme patterns are pretty advanced for his era. Still, G Rap had great rhyme patterns at the same time and is a hardcore rap pioneer.

Ace influenced cats like Eminem and probably many more

But G Rap has a legion of great emcees who either directly cite him as an influence (Pun) or who folks generally consider to be influenced by him (Nas, and indirectly everyone influenced by Nas' style).

Can Ace be credited with the first concept album with Slaughterhouse or did someone beat him to it?

Peace,

Spirit (Alan)
http://wutangbook.com