If I remember correctly, The 99 400 is a two stroke. Do NOT remove the silencer. It will lose power and be very loud and could cause engine damage if it causes a lean condition. If you want more power or better sound, get a good pipe/silencer and dont forget the jetting when you do so.
With a 2stroke it is very important to have an exhaust, it needs a certain amount of back pressure, also you will lose a TON of performance without an exhaust. much more important then a 4stroke exhaust.
So yes, keep the exhaust. is there a reason you want to get ride of it?