Then you will only get around 100 mmols/m2. Will probably work, but I feel more and more after running alot of numbers of well run tanks that 120 mmols/m2 at the bottom is the magic threshold for good pearling and the ability to hold down H. callitriciodes for example.
Going down to 80 mmols/m2 is definitely pushing it down where you will limit yourself plant wise.
If I'm not mistaken some high light plants have their light compensation point at about 80.

(Oh. I'm talking PUR-values here btw. PAR-wise you will get 200 mmol/m2 with 54 watts and over 240 with 68 watts.)