I am unable in any way to come up with a 25 dmg per swing benefit to gthf so i'm gonna go over it all again. Turns out you have some multiplication errors in your original numbers.
Let's consider your original math argument again.
Instead of caluclating out a 20 swing average which is just prone to errors like your multiplications and gives a bloated number for an average dmg per swing calculation let's take a look at a single per swing dmg average for a bastard sword with Overwhelming Critical. This all assumes hit on 2 or above while not idealistic it is simplistic and attack bonus should not change with either choice significanltly.
Gthf Base attack seeker 10 assumed.
.75(x)+.1(2(x+10))+.1(3(x+10))
or simply 1.25*x+5.0 where x is base dmg.
Glancing blows then calculate out to.
.95*.75(.5(x))
or simply .35625*x
So a gthf attack would do on average 1.60625*x+5.0
Now for a 5% double strike calculation:
Base attack
1.05(.75(x)+.1(2(x+10))+.1(3(x+10)))
or simply 1.3125*x+5.25
Glancing blows on a imp thf setup would be
.95*.5(.4(x)) or simply .19*x
bringing out total to 1.5025*x+5.25
For a difference of
.10375*x-.25 per swing.
If we let x = 50 for a moderate amount (10 for 2[1d10], 6 enhancement, 5 power attack, 20 str mod, 9 additional weapon dmg bonuses) we get a difference of 4.93750 per swing.
I've already shown how divine sacrifice can drop this to ~3.75 dmg per swing. Other attacks like lay waste/momentum swing that carry with them expanded crit range/multipliers will lower this also along with attacks like volcano's edge/confront any foe that grant additional non base dmg effects..
Then theres the whole concept that noone uses a plain bastard sword and we have additional magical effects to consider.
Magical effects break down as follows. For non crit based effects the doublestrike setup breaks out at 1.05y for regular attacks + another .06(.5y)=.03y for a total of 1.08y
The gthf setup gets 1y for regular attacks and .09(.75y)=.0675y for a difference of .0125y.
For crit based magical effects imp shield mastery case gains .05(.2)z=.01z. y=attack based magical weapon effects, z=crit based magical weapon effects. Vorpal only affects also give a .0025v benefit to the double strike.
This isn't gonna amount to a ton but it does add up when you factor in weapon flags, sneak attack dmg, things like sense weakness etc. and depending on your setup can create a virtual push.
I won't even go into the benefit Nightmare gets with double strike which happens to be the preferred and most often used weapon for a build of this type.
Then theres the whole movement thing that throws it all out of whack in favor of the imp shield mastery plus you also get a fair amount of prr.