I've been running mine at
PSI - 135
No spacers/spacer body (I don't even own any inserts/spacers at all)
Blue Flow Insert
Don't know about efficiency numbers but the gun shot nicely all day.
This had me right around 290 fps +/-6. Not sure how broken in my internals are, haven't had the gun in my possesion long.
In short, no, you don't have to have the spacer body in the gun for it to operate but you should notice an fps increase by up to 20 when you take it out and keep your settings the same.