Many Mercury Owners/Enthusiasts are interested in making their engines breathe better, and inquire about exhaust systems geared to maximum power and/or overall performance.
Here is a table that shows the best pipe diameters for your engine:

1st Column = Pipe Diameter (inches)
2nd Column = Pipe Area (square ins.)
3rd Column = Total CFM (estimated)
4th Column = Max HP Per Pipe
5th Column = Max HP For A Dual Pipe System
1 1/2" -----1.48 171 78 155
1 5/8" -----1.77 203 92 185
1 3/4" -----2.07 239 108 217
2" --------- 2.76 318 144 289
2 1/4"----- 3.55 408 185 371
2 1/2"------4.43 509 232 463
2 3/4"------5.41 622 283 566
3"---------- 6.49 747 339 679
3 1/4"------7.67 882 401 802
3 1/2"------8.95 1029 468 935

Estimated figures based on Outside Diameter, 16-gauge pipe thickness.
Too small of a pipe robs top-end power.
Too large of a pipe decreases low and mid-range torque.