Distance between HPS lights

5'x15' flowering room. HPS lights. Entire area covered fully in mylar. I see 1000w ballasts and 400w ballasts aren't that far apart in price.

Would it be benificial instead of buying a 1000w ballast , to buy 2 400w and spread them apart? Worth the cost of an extra ballast? Will the mylar spread the light artound well enough?

The Idea is to get 10,000 lumens to the plants.
A 1000w HPS @ 4ft=10,000 lumens
a 600w HPS @ 3ft=10,000 lumens
a 400w HPS @ 2.25 ft = 10,000 lumens (taken from Gardening Indoors)

Using more lower wattage lamps is more efficient in lighting and per Kw cost however is more expensive to set up initially. if you buy your lights used though its not much problem.
Don't forget you can take advantage of a light mover too.

One last note, if our buying lights buy horizontal bulbs rather than vertical as they are more efficient.

I heard using 2 six hundreds instead of 1 1000w is a more efficient way of growing concerning the lumens. Read it in one of george cervantes's books