The idea works and is the basis for merge collectors that I install on racing L6 exhaust systems: 
  
 
  
But it is ABSOLUTELY NOT a one size fits all modification.  Engine displacement, cam, head flow, header design, etc. are all important considerations when designing a merge collector.  Done right and you can improve mid-range and/or top end full throttle power without affecting the rest of the power band.  Done wrong and you will reduce power across the board, create a big flat spot in the power band, or help mid-range and kill top end power.  The item you link to above is basically a restriction in the exhaust pipe and not worth spending a dime on.