Length of the header doesn't make a pipe illegal. I'm not familiar with the MAX pipe.
Changing the restrictor changes how much air goes into the carb (ins box does the same thing in a less controlled way). A smaller restrictor will reduce the amount of air, thus a little less top end but better fuel efficency. A bigger restrictor will increase the amount of air, thus a little more top end but less fuel efficency.
As far as staying away from the INS box because most tracks don't require it. I wouldn't bet on that for much longer. Most of the big races are starting to require them, and it will probably be a rule within two years for 1/8 scale (it already is in Europe). Oh yeah, the newer Novarossi engines (or any European engine manf.) are designed around the INS box and a 3 chamber pipe. So you will actually get the full performance of the engine.