Wow! Had no clue Stevie played. That's awesome. Always loved his music. I've seen vids of Sammy playing before. All around talented guy.
As far as under rated or fairly unknown, I'll just throw Jon Fishman in the ring from Phish. Besides being a great jazz drummer, he plays songs that jam on from 8-20 minutes or more. Great chops, not overly "show-ee". I'm quite biased. I've been a Phish fan for nearly 3 decades now.
Kenny Arinoff was "unknown" for the longest time. Then BAM! Everyone wanted him on their album. I met Kenny in our hometown in a record store the first time in '92. Got his autograph. Then a few years later I hung out with him and another friend as I was DJ'ing in a local restaurant/bar. He rocks! Great guy. My best friend took lessons from Kenny in the late 70's for a few months.