Ok, we have quite a few keyboard mechanics here it seems. The muffler is a source of back pressure. That back pressure is part of the highly optimized emissions systems tuning done by the manufacturer. When you remove that back pressure there is high likelyhood that the reading from the 02 sensor are going to change and cause the computer to alter the air-fuel ratio based on that change. Hence, you will likely lose power, mpg, and introduce the possibility of misfires and other unwanted behavior.

I saw this analyzed on a Chevy Silverado that would randomly misfire. Everything else checked out: fuel pressure, spark plugs, firing timing, and fuel injector pressure. The remaining item: muffler delete.