Performance should be simular since the block is nearly the same. I believe the 115 has 118.9ci block vs. 124.7ci block on the 140. Also the gear ratio is higher 2.38 vs. 2.59 on the 140 so a boat running the same prop should have a higher speed at the same rpm. My twin 115's top out at 6000 and the 140 would top out at 6200. Sorry I don't have realworld #'s for the 20 but I hope this helps. My simple gues would be 2-3mph difference max. Here is the link to Suzuki
http://suzukimarine.com/sr06/df140/f..._and_specs.php