I'm going to be towing as well...over Colorado's high passes so torque is also supremely important to me. I've done hours and hours of research on this issue to see how I can optimize the torque on my Tundra in that all-important-for-towing 2500 to 3500 RPM range.

From my research, I've concluded the key to getting maximum mid range torque is to maximize exhaust energy (heat) and velocity all the way to the end of the tailpipe...at the exhaust volumes and flow rates the engine produces in the mid range.

Furthermore, if possible, the pulses (from the exhaust valve openings) in the exhaust flow should be maintained...the low pressure on the back edge of one pulse helps pull along the high pressure on the leading edge of the next pulse. This is the principal of a scavenging exhaust.

The reason that dual exhausts...or overly large (like 2.5 inch and larger) single exhausts reduce low to mid range torque is they allow the exhaust gasses to expand (which cools them) and to lose velocity. The cooled air is denser/heavier...and the exhaust pulses more or less are merged together further losing their velocity and scavenging action. The net result is the larger exhaust pipes become essentially long tanks of semi-stagnant gas which has to be pushed out by the incoming exhaust...it's no longer self-scavenging. Or, in other words, a larger or dual exhaust has higher effective backpressure than a smaller, single exhaust at mid-range RPM . Yes, the larger/dual pipes have less wall friction on the gasses than the smaller single pipes but this effect is nullified by the loss of energy/velocity in the larger or dual pipes. It's very important to note this situation applies to the mid-RPM ranges...at very high RPM (like near redline), there's enough energy, volume, and velocity in the exhaust gasses to maintain scavenging all the way to the end of the pipes (even duals) and the lower wall friction of the larger pipes does come into play.

Bottom line: for mid RPM ranges, intuition is wrong...there's more effective back pressure (or less scavenging) in large or dual exhausts; for top end RPM, intuition is right...large/dual exhausts have lower backpressure. And at any RPM, backpressure is just plain bad for torque. There's a myth out there that engines need backpressure to make more torque. This is wrong. Backpressure is resistance and you want the least resistance. If you look at the dyno results and comments posted by V8toilet about his various systems you'll see that he got noticeably better mid range torque than stock by using a single 2.25 pipe running through a fairly low restriction Spintech muffler...and much, much better mid range torque than with either 2.5 inch single pipe or with dual exhausts.