Hmm... Ive found that it doesnt actually reflect that much light, as the bottom canopy is quite thick anyway. If you tie your plants down (LST) then it could be beneficial. Its something that youll have to experiment with really. Also if moneys on a budget, then dont measure the whole of the growroom. Put the pots in that youre gonna use (the largest size, if you transplant. I just stick them in the biggest pot and leave them in there til harvest, I cant be arsed with transplanting all the damn time) and only measure from the pots upwards.
Like elcheapo said, more reflectivity the better, but is forking out an extra tenner so you can cover the floor in it going to pay off with more grams of bud?