According to Baseball Savant, Martin's throws average out to 85.3 MPH, but that is based on OF throws. However, that number is higher than Kirilloff, Larnach and Margot in the OF, so he may become a serviceable LF or 4th OF.
The average MLB 2B throws 79.1 MPH, so Julien's 80.2 MPH is slightly above average and higher than former SS Semien, IK-F, Torres and Bogaerts as well as Altuve and Albies.
Other than arm strength (MPH), I am not a big believer in advanced metrics for fielding because there are too many variables to take in (positioning, jump, exit velocity, field condition, game conditions, pitcher and batter handedness, pitch velocity, pitcher's control that day (striking out or walking a lot of batters), game score, runners on base, etc...). I tend to believe what my eye sees (which, thanks to MLB and RSNs is getting harder to do) concerning glovework, footwork, positioning, quick (or not) hands and situational awareness etc...