That 'sewing machine' noise is more than likely valve train noise- its normal, both of my bikes make it with aftermarket mufflers. It was probably making the same noise before you removed the baffles, but the exhaust sound/frequency covered it up.
Harley puts a lot of R&D into getting their exhausts and engines to sound just 'right'... They have an underground warehouse sized anechoic chamber and a staff of acoustic engineers that test and tune the exhausts.

Loss of low end torque--- Its more likely that you are putting your bike out of perfect tune by opening you exhaust than anything else- its either not getting enough fuel or you messed up the exhaust scavenging at lower RPMs. It should perform better in another rpm range.
Some 'back pressure' is necessary on 2-smoke engines to run, but has no positives whatsoever on 4-strokes. Ideally, you'd have a 'vacuum' on the exhaust side sucking the burned gasses out and beginning to draw fuel/air mix on the overlap where the exhaust and intake valves are opened at the same time. You get a bit of 'vacuum' when the last exhaust pulse runs down the tube and pulls the next behind it. removing the baffles or changing the pipes changes the optimal timing between pulses or RPM range.

 

Yes, it is normal, but not necessary. Check out the Rocker Lockers at this LINK, they do a great job at reducing top end noise.

The best way to prevent reversion is not to put a baffle in which creates power robbing back-pressure, it is to use a Patented TTI that does not mess with the tune, does not rob power, and prevents power robbing reversion.

 

I am going to look into those TT inserts. They seem like a properly manufactured version of the lollipop baffles my dad and all his buddies have ran in their pipes since before I was thought of.

 

Yes, I used lollipops for over 4 decades, and the TTI's are a highly refined lollipop that we came up with after thousands of hours of development. Lollipops definitely work, and the TTI's work many times better.