The only reason to install an INS-BOX is to comply with the rules set by the race organizer. They are designed to reduce the noise from the air intake. The noise produced is indeed a bit milder, but not much. In have not noticed any power loss, but I don't run back-to-back with and without the INS-BOX, so there might be a very slight power loss. Other than that, it adds weight to your car where you don't want it (sitting high up).
All in all, the biggest advantage of running an INS-BOX other than making your car race legal is that your air filter collects much less dust, thus requiring less cleaning.
If you have to use one, there are three INS-BOXES that actually fit a 10th scale. The Mugen box has expensive and crappy airfilters, I used it and would not recommend it. I use the Novarossi box which is good. Same box is also available from RB if you fancy the orange color. Then there is the Xceed box, which I haven't used or seen used locally, put pics from the big races show that quite a lot of the pro's use it.
Shepherd V10 Pro 2015 - Matrix tires - Novarossi Engines - Runnertime