When growing pot lighting is 90% of the battle to get the biggest buds,not just good or smokeable buds but big fat ass buds that make you smile every time you look at them all over the plant.
Aside from getting a larger light throw with the 400 watt vs the 250 watt you get increased lumens,the higher the lumen output the deeper into the grow the light will penetrate,the farther the light penetrates the larger the buds will grow every single time.
When you get proper light penetration with intense lumen output to the lower levels of the plants it turns what would end up being small light & fluffy buds into large rock hard nuggs that are as high in quality as the main cola of the plant.
Another aspect of the larger wattage lights is light intensity,lets say you start off with a 250 watt light, then after a few grows you decide that you want more light for added grow space so you add another 250 watt light to the grow op,even though the two lights combigned wattage would be 500 watts they would not match the light intensity from a single 400 watt light,lumens do not stack up like alot of people think they do,the light intensity is the lights ability to deeply penetrate the canopy & remain at effective output levels at all areas of the grow including the lower levels.
Aside from getting larger denser buds with the more powerfull lights there are other benifits as well,one benifit is again focused on the lower portions of the plant only this time its the leaf surface area,by being able to bring effective levels of light to the lower leaves it allows them to do their job properly & send more energy to the plant through photosynthesis,if these lower leaves are not getting enough light they end up doing the plant very little good & die off much quicker than what they should.
For maximum plant health through out the durration of the grow you want the plant to retain as many sun leaves as possible,once the plants are in heavy bud afer a month or so of 12/12 is when the plant needs all the sun leaves it can get,when the plants start to reach full maturity its natural for the sun leaves to be eaten alive by the plant & die off,you want as many healthy sun leaves as possible when the plant reaches the point where it is in heavy bud so those leaves can fuel the plant,having intense light penetration reach the lower sun leaves will allow them to live longer & continue to help pack on bud weight.
I know it was a long answer but i hate short answers because they dont explain much,your yeilds will be a pretty large increase over the 250 watt light,i'd estimate a full 20% increase in yeild.
