My take on the headers/carb upsize is that all you do is move the torque peak higher in rpms, thus since hp is a function of torque x rpm, you raise the hp, but into a less useful band for what he wants, which is low end torque. I have a bet with myself that a lot of the gain going to headers on a 2.6 is losing a partially blocked cat converter.

The v6 and the 2.6 both make around the same torque (about 150ftlbs, IIRC) but differ in hp, in large part due to the better breathing of the v6 fi motor at higher rpms. The 2.6 is built for torque, with an undersquare design (stoke longer than bore with a long crank throw for more leverage0, and the v6 is built for revs with an oversquare design. They bandaid this with intake runners tuned for low end torque to give a pretty flat, wide torque curve.

The turbo makes markedly better torque, and as Kevin says, it comes in early. The small turbo spools fast. In 4wlo, you should be able to push all the traction will handle with a snowplow. I pulled a 4k lb boat/trailer with a turbo 2.6 in an 87 monty a/t with no trouble, except for getting it stopped in a hurry.

If you want to make more power, you have to get more air/fuel through the motor per unit time. Air is the hard part. Turbo rams the air in there better than any other solution. Headers/cam/carb mods are like putting a better edge on your axe so you can cut down the tree better. A turbo is like switching to a chain saw...