That is a good analysis to compare it to Weapon Proficiency (some superior weapon). In my opinion, though, superior weapons are also slightly broken. ;}
I thought so too until I took a look at the mathematical difference between a superior weapon and the equivalent non-superior weapon of the same type.
Battleaxe vs. War axe: You get a d12 damage die. So that's an extra point of damage per [w] on average.
Flail vs Triple headed flail: You get a +1 to hit.
Warhammer vs Craghammer: You get brutal 2, effectively giving you d8+2 rather than d10. Average damage of 6.5 rather than 5.5. An average of +1 damage per [W].
Great Ave vs. Execution Axe: You gain Brutal 2. d12 brutal 2 is mathematically identical to d10+2. The average damage of a d12 is 6.5. The average damage of d10+2 is 7.5. So it nets you an extra point of damage on average per [W] of the attack. Both are high crit.
Greatsword vs Fullblade: You gain high crit and d12 damage. On non critical hits, the d12 has an average damage of 6.5. The d10, 5.5. So once again, you've got a +1 per [w], this time with a maximum that's higher by 2 (10 vs 12 possible points of damage). Add in the high crit and in this case, you're getting a decent bang for your buck for the superior weapon feat-- average of 1 more point of damage, 2 more possible points of damage and high crit.
Longspear vs Greatspear: You gain +1 hit.
Maul vs Mordenkrad: You gain Brutal 1. That means you have the mathematical equivilant of 2d5+2 rather than 2d6. So that's 8 damage on average rather than 7. +1 damage per [W] just like the Execution Axe.
Double weapons generally get you an increase in damage die for your off hand weapon only, as well as the AC boost. So a two weapon ranger gains a bonus AC point, but nothing really changes as far as the damage die goes (the ranger can already use full size weapons in the off hand). For the tempest fighter who doesn't multiclass, you gain the AC and your offhand goes from a d6 to a d8 or a d10. That's an average of 1 or 2 extra points of damage per [w], but only on the off hand.
For a lot of these, you might be better off taking weapon focus and getting +1 to damage per tier. Unless you make a lot of 2[W] and greater attacks. Then +1 per [W] is better.
At the heroic tier, these are all good options, not quite on par with expertise. But they don't scale at level 15. From levels 1-14, going from a long spear to a great spear or from a broad sword to a bastard sword or from a flail to a triple flail are pretty much the same as Expertise as long as you don't switch weapons. If you switch weapons within a single weapon group, expertise is better.