To me Greenway looks like the type of guy who is ready to play NFL 3-4 OLB right away. As far as his "measured speed," that does NOT translate to how fast he plays on the field. From what I've seen of him he looks very fast on the field, and has more than enough "competitive speed" to play the position.
Bottom line, he looks like a football player, and has great size, and prior to the combine, everyone felt really good speed (based on how he plays in a real game). I think there's a real good chance he'll be there at 16, and he will definitely be on Saban's radar...
The guy can rush, cover, and defend at point of attack --- not bad.
However, I'm not sure he will turn into a "Monster," and if possible, it would be great for us to get a "Monster" in the 1st round. Perhaps Simms has more upside, and more "Monster" potential. The guy who has the most "Monster" potential (Manny) also has the biggest downside risk (i.e., development time, unproven OLB skills, etc.).