I know that the horespower gains are very minimal but comparatively speaking. Which gives better horespower gains:

1. A Two Brothers slip on exhaust or
2 A Full Muzzy two into one exhaust.

I'd say a full exhaust gives you more horsepower. I've been wrong before though.

Generally with a full exhaust the pipes are bigger and/or they bend, and/or collect in different areas and are better tuned to the engine. With these improvements you can not only see more horsepower, but what I like to refer to as "better" horsepower. Mostly I think your biggest gain with the full exhaust will be in torque. Let's not get me started on a torque vs horsepower debate.

For what it's worth, AreaP sees zero hp gain for a slip-on alone on the ninjette, which is why they recommend the full system.

If you just want a little more sound and little more midrange I've heard that you can drill out the rivets on the end of the pipe slide out the center tube and put 4 holes in it going in cylindrically. If you do this to both pipes you won't have to rejet. Just check that your carbs are synced and that the mixture is correct.