the n249 bypass is a simple mod which just eliminates the ecu dumping boost at low throttle and sometimes it becomes faulty and starts dumping boost randomly and can cause jerky pull off. also it helps hold boost for longer. the n112 and sai bypass/ removal are a bit more complex and all require resistors ect.. to bypass. if after doing the n249 bypass you still experience boost issues then maybe a n75/n112 and sai delete is the answer but in my opinion keep it simple and just do the n249 unless you believe or know the rest to be at fault.
also weather or not your car is standard or modified the n249 bypass works but its best used on a uprated dump valve as the n249 is a protective measure against the stock dv failing.
This pretty much covers it, just done mine and it is smoother and holds boost longer....