Exhaust tuning is a black art. I have ridden bikes with aftermarket exhausts that worsened performance. That was a long while ago, though. If you buy a pipe from one of the good manufacturers designed for your bike, you should get improved acceleration. Even a good pipe is not magic. It takes a fair wad of cash to get a relatively small performance improvement. Remember that the Maj will always have a 400 cc single cylinder engine. I think that a lot of guys really get new pipes for the look and sound. You will probably get worse gas mileage, too.
The Majesty exhaust note is not bad as it stands. It is about as loud as most state laws allow - much louder than my other, much larger bike.
In many states, the police will look for DOT approval on aftermarket pipes. Of course, DOT approval is not found on aftermarket pipes, so you get a ticket. This usually happens when the rider is pulled over for speeding and then is discourteous to the LEO, but sometimes they nab you in a gas station.
If I were trying to improve my performance, I would look at tuning the variator first. Not so sexy, perhaps, but much better bang for the buck. Apparently you can swap weights and springs on the stock variator, or go for an aftermarket variator.