Not so much HP, but torque, yes! Take a look at any multi-cylinder vehicle and they all have a system that joins the pipes together. That is not to cut the cost of making the exhaust system, but to get the benefits of inter-action between the cylinders. The classic is a 3-cylinder 2-stroke, which with a 3-into-1 system can produce seriously more torque than any other configuration.

Our V-twins are not as good, but they still benefit from siamesing the pipes. Where they join is important and if both pipes are the same length the benefit will be even greater. For touring bikes there are a couple of brands that achieve that, but it involves some serious gymnastics with the rear pipe.

Buells all used equal length pipes and were able to use much higher CR, wilder cam profiles and get far more performance, HP and TQ, than a Sportster ever will.