Wulf Ratbane
Adventurer
OK, so just to be sure I'm getting this straight:
A Ranger (or any fighter with 2-wpn fighting) would look like this:
6th level (2-wpn fighting)
+2/+2/+2/+2 (+6 BAB, -2 to attack for multiple attacks, -2 for 2-wpn fighting, extra off-hand attack due to Improved 2-wpn fighting)
11th level (assuming no adjusted greater 2-wpn fighting)
+8/+8/+8/+8 (+11 BAB, -1 to attack for multiple attacks, -2 for 2-wpn fighting, extra off-hand attack due to Improved 2-wpn fighting)
16th level (same asumptions)
+14/+14/+14/+14 (+16 BAB, -0 to attack for multiple attacks, -2 for 2-wpn fighting, extra off-hand attack due to Improved 2-wpn fighting)
Incidentally, I like the idea of altering Greater and Superior TWF in the manner you propose.
It's early and pre-coffee, but that looks right.
As you mention, with the alteration to GTWF and STWF, he'd be +16/+16/+16/+16 at 16th level.
One more +16 with haste.

All attack rolls always at the same attack modifier-- that's the important point. If they're not, you (or I) messed up somewhere.
The ranger is easy. I really should review this against the monk. (This is much closer to the monk's attack progression.)