When I start onions from seed in a flat, every few weeks after emergence I trim the growth back to prevent them from falling over from too much top growth and help sturdy them up but have never thought of trimming the leaves once they were planted and growing.  Might have to give it a try, the young seedlings definitely seem to respond well to being clipped so it may work with larger plants.  If it makes them produce more leaves I guess it could translate to larger bulbs but a downside could be possibly opening them up to disease or the constant re-generation of top growth could interfere with bulb growth would be the unknowns I can see.