I've already mentioned it in another thread, but maybe it could be balanced a bit if STR didn't give extra HP, and if IF was decoupled from STR. You could then make a 10 IF AGI char.
You'd have to increase significantly the amount of HP given by IF, otherwise you'd end up with only 55 hp even with 10 IF and people would die too fast. Hybrid builds which can't put many points into IF would also be left with between 35 and 50 hp at most (if IF gave 4 hp per point for example) while STR builds (and other pure builds), which only have to put points into PS and a few into ath, would still be able to afford a lot of IF and would still have much more hp than other builds. It just isn't a solution imo.
Regarding WM, you forgot to take the WPF reduction from armor into account. WPF has the most effect from 1 to 100 iirc (below 100 you actually deal less damage than normal), but a STR build (which obviously usually uses heavy armor) has barely 110 without any WPF, so he'll lose a lot of WPF due to this armor. For example, with playing with byrnie and light mail on my 36/3 STR build with 111 wpf, I've got about 100 wpf after the reduction from armor. When playing with the heaviest gear available (gothic with bevor, weimar helmet, heavy gauntlets & black greaves) it goes down to 60. And then I do see a big difference in speed and a slight one in damage.
Your agi build with 180 WPF probably wouldn't be using heavy armor, so you'd probably still have about 160-170 effective WPF. That's 100 more than the STR build in armor, so the speed bonus is much better and the agi build gains damage equal to about 2 PS.
Imo WPF is important for STR builds as long as you're using heavy armor, so that you still have about 100 effective WPF, if you're playing with light / medium armor it's probably not worth it. Agi builds do get a lot more speed if the STR build is playing in heavy armor (which is the case for most of them).
You're also not taking into account the fact that agi builds are much more polyvalent than str builds. STR does give you both damage & hit points, but that's the only things it gives you. You need STR for both PD & PT, but no one would play an archer / thrower with pure STR because you need a lot of WM (and thus agi) to hit anything. On the other hand, agi gives access to a lot of other roles (that pure STR can't do, but that pure agi & balanced builds can) like shielder, cavalry, archer, thrower, xbowman (because of the WPF requirements), hybrid builds (you won't have enough WPF for 2 weapons with 1 or 2 WM) and of course anything that requires speed (like rageball runner). The only role of a pure STR build will be to fight with the mob and do the killing (he's too slow to do anything else and will probably get gang banged alone) but he's obviously incredibly effective at it. Balanced builds will have a lot of obvious roles through shielders, cav, archers, etc... They're also able to play solo in battle and get away from tough situations while STR aren't.
The issue at the moment imo is that there aren't a lot of things that pure agi builds can do that is much better than what balanced builds would do. They're good as 13 ath runners in rageball or 13 shield trolls but that's about all. I don't think buffing WM would help them a lot, it'd probably benefit balanced builds much more.