You should also consider the weights of the 2 vehicles. Your GS is 3900lb whereas the quad cab tundra weighs 5155lb and 4900lb for regular cab. 1000lb difference can also be viewed as a 100bhp difference. Also, your GS has an 8 speed auto w/ 150mpg top speed, so if the tundra has a lower top speed, you'll have him there. The extra gear changes might sap some time but its hard to say.. never driven the new 8-speeder! Another MAJOR factor I just thought of is your drag coefficient. The GS has a VERY low coefficient of 0.27. Trucks have horrible drag coefficients due to their design so he's going to need more raw hp to fight wind resistance. I say its up to the driver?
340hp versus 500 = 160hp defecit so in all honesty I say it will be close w/ the variables above adressed.