You need back pressure because of the valve overlap these high revving engines have. It helps with exhaust scavenging. So there's a vacuum created from the higher velocity exhaust leaving the pipe (from the stroke before) that helps to pull the exhaust out of the cylinder on the current stroke.

As the intake valves starts to open, there's now increased pressure around the exhaust valve which helps the intake charge stay in the cylinder despite the exhaust valve being open.

So, its a balancing act. Too much back pressure = bad. Too little = bad.

 

If I didnt have a full yoshi exhaust id be all over that exhaust

 

I completely forgot about this thread. I never included pics of Robin! :O

 

That's the exhaust. I bought the bike in September of last year with only 92 miles. She's about to roll over 10k