Exactly what I was looking for. Thanks.Generally speaking,That said, reducing weight also improves performance and the Ninja 400's stock silencer is bulky and heavy, so installing a slip-on is a weight-reducing modification. Installing a full system will reduce weight even more.
upgrading to a slip-on affects sound rather than performance and upgrading other parts of the exhaust that are ahead of the silencer affects performance rather than sound.
The reason a slip-on doesn't improve performance very much is because the silencer is positioned beyond the portion of the exhaust system where restriction and/or exhaust flow has the most potential for improvement.
Upgrading to a full system will potentially reduce restriction and improve flow (these are not the same thing), particularly in the "midpipe" portion of the exhaust. A full system will usually eliminate the catalytic converter, which may or may not be legally permissible where you live.
@Watcher905 is correct about an ECU piggyback device or a reflash being required to achieve full performance.
Consider also that you want to avoid a lean condition. Bikes tend to run lean from the factory, and de-restricting the exhaust and/or the intake can lean things out even more. For some bikes the lean condition is a critical concern and for others it's less of a concern. I'm not qualified to say how important this is for the Ninja 400, but others on the forum probably are.
IMHO a slip-on is a compelling mod because it improves sound, reduces weight, and looks good. A full system is less compelling because the Ninja 400 stock exhaust seems to be better than most, and replacing it contributes to a lean condition and I'd prefer to avoid that.
I'm lurking around waiting for some feedback from BoosterPlug pioneers.
Removing the airbox snorkels is another mod to consider.
Also related: PAIR valve removal