the elbow is the restriction on the 2wd one, i think 4wd can flow up to 450iirc, the bigger the spacer the more power in theory but you will get a small increase in lag as the plenum chamber is bigger (hardly noticable for me), if you fit one its more simple to use a 14mm spacer as you dont have to modify the fuel rail mountings you just bolt it to the other side of the mounting,