That's simply not true:
http://www.fangraphs.com/pitchfx.aspx?playerid=14165&position=P
His max FA or SI velocity has been 90.8, his average velocity 88.8 and 88.6. His FA has been 86.7-90.7 MPH. He hasn't thrown a single MLB pitch over 91 MPH, much less 92-93. Do you think he has been less pumped up or saving himself more than usual at the MLB level? Do you think he was throwing harder in the minors than in Atlanta?
His sinker has an average horizontal movement of -8.2 inches, an average vertical movement of 1.2 inches, and an average velocity of 88.6 MPH.
According to this:
http://www.fangraphs.com/fantasy/the-change-average-movement-for-each-pitch-type/
the average MLB sinker has movements of -8.4 and 4.6 with an average velocity of 90.9 MPH.
Rob Whalen's sinker moves several inches less and is more than 2 MPH slower than the average MLB sinker. There is nothing to suggest it is a "very good sinker". It is, quite literally, a below average sinker in every measurable way.
I keep bringing up 25 year olds because the discussion has been over a 3 year window, when he will be 25. Pitcher velocity almost always decreases with age, so by age 25 he will most likely be throwing even slower than 88.6 MPH.