the GSR and LS have completely different gearing. yes the LS engines might have more low end torque but the gearing changes that torque value when calculated at the wheels.

the reason horsepower is more valid is because it's calculated from both torque and engine RPM. the inclusion of engine rpm in the equation allows for gearing to be considered in the number, making it more valid than a peak torque number.

a smaller engine needednt work twice as hard, merely twice as fast. it's just like anything else.

[spoiler=analogy.. please read if you really don't get it]
heres an analogy. you are sitting at a table with your little brother. you are 10 and he is 4, there is a bowl infront of you with 4 peices of candy in it.

his hands are small, so he can only grab one peice at a time. you have bigger hands so you can grab two. imagine this as your "candy torque"

now imagine he is twice as fast as you are. this is the "candy RPM"
you grab 2 peices as he grabs one.
he grabs another.
there is now no more candy left and you are both left with two peices.
[/spoiler]
what i am saying is not that a small engine must work twice as hard, but i'm saying if it does it will be able to make up for the loss in torque. it's the reason why it's inconsequential to just know and compare torque values.

not that it is perfect comparing horsepower values, but they are much more correct than torque values. the absoloute would be the integral of the torque curve from 0 to the maximum rpm.

using horsepower is just a shortcut which assumes many things to be true because technicians dont want to deal with calculating integrals and the area under a curve. and gear ratios...