most all conventional ballast will have atleast a 10 percent or possibly more efficiency loss but even then they are still one of the most efficient lights available for the amount of light they put out.
if you do decide to get a larger light you may consider a more efficient electronic ballast, going by your room size alone you could use a larger wattage light if you intend to flower plants indoors with that room size but if you are just vegging some smaller plants underneath it to move outside later or to another flowering room your 150 should be fine. the reason they use room size is so you can use the reflected light from the walls by attaching mylar or other reflective material to the walls they will not be wasting any light that could benefit the plants.
to get the most out of your 150 watt light you can also put up a temporary wall to reflect light back to your plants it doesnt need to be any taller than your grow light height would ever be set at, its not always feasible to do all 4 sides and in larger grows it is often more conveinient to have the space for maintaining and getting to all the plants than what it is worth to use walls at all.