When you are talking about yield, it will always be a very hard question to answer. There are just too many variables. Are these being grown from seed or clone, how long of a veg, the strain you are growing, and the conditions you provide.
with that said, it is easy to say that I get this amount per plant so you sould be about to get that as well, but this is only true if you are growing the same or similar strains in the same or similar conditions. So yes it is easy to get 2 oz's per plant growing a small number of larger plants, but it is a lot harder to achieve this same figure, if you are loading a large amount of clones, because the plants will have to be smaller.
Also another major aspect is the space and lighting efficiency. So you can have a 400w hps in different square footages and it will yield totally different results.
A good rule of thumb to shoot for is 1 gram per square foot. In an efficient garden with an experienced grower this can be achieved regularly, but starting out you will see yields closer to .3-.6 grams per watt, so around 120 - 240 grams total, from your 400w light.