Hard Tubing is measured using outside diameter. Flexible tubing and rubber hoses is measured using inside diameter. The reason the Y-Pipe is so small is the keep the exhaust pulses slow enough to keep up a certain amount of back pressure to maintain torque. Once the pulses reach the larger diameter pipes they speed up which help produce horsepower. As far as getting 20 ft-lbs of torque with a slip-on, who knows. I'm sure it can be done with the correct diameter and length tubing. But typically, we the common people do not have the means to engineer such systems (computers and dynos), which is why we purchase aftermarket items from a dedicated company.

Exhaust systems are a huge balancing act, from manifold to tail pipe. Sure you can get a huge horsepower increase but it will come at the sacrafice of torque, and vice versa. Most street applications are designed for torque. With such a small engine (3.3L), I can't say that 20 ft-lbs with a slip-on is not achieveable, but it certainly can be reached with proper sized pipe and muffler, but probably at the sacrafice of top-end horsepower. Just another opinion.