I think DC Heroes inflated the values of a lot of their characters, but this is probably due to the lack of fine detail and wanting to make the characters feel right in game terms (STR 18 in M&M would be STR 3.6 in DC which they call STR 4). Using a literal translation, Batman should have a STR of 25 in M&M and Nightwing would have a STR of 20.
M&M follows the doubling mechanic to a certain degree (Telekinesis 11 gives you a lifting capacity twice that of Telekinesis 10), but doesn't define the differences for all characteristics. STR 25 is twice as strong as STR 20, but I'm not sure that INT 25 is twice as smart as INT 20.
Regarding my Attack and Defense benchmarks, I didn't mean to use skill rank for all skills, only combat skills like Martial Artist or Weaponry since they can substitute for DEX. So when I wrote "(DC DEX or Skill AP)*2 - 4" I only meant MA or Weaponry skill rank.
E.g., if a character had DEX 8, Martial Artist 9, I'd give him a base attack or defense bonus of +12 (DEX 8*2 - 4) and two ranks of Attack Focus (melee) or something similar since I modeled each AP as representing +2 in M&M (or alternately, I could have computed attack bonus as MA 9*2 - 4 = 14).
I don't have the DC Heroes rulebook anymore, but if I remember right, MA could substitute for AV, OV, EV, or RV or maybe DEX instead of AV/OV. The two ranks of Attack Focus could have been two ranks of Dodge Focus or other feats like Power Attack or Defensive Roll. Since the two systems are based on different mechanics, I'm just trying to get in the right ballpark and then add whatever feats seem appropriate.
I don't think my benchmarks are very accurate, but I'm trying to come up with something that feels consistent. Basically I have DC DEX 2 set as +0 Attack and Defense and DC DEX 10 set as +15 or +16. The parts I'm unsatisfied with is what to do with DEX or Weaponry ranks greater than 10. I don't really think Green Arrow's Attack Bonus with a bow should be higher than Batman's Attack Bonus with melee. I'd probably give Green Arrow a +15 with the bow and Batman +15 or +16 with melee.