how tall is the tank? I've seen a lot of people do this before with lighting fixtures and make out ok.. you just gotta realize that you can't have the high light corals on the sides obviously. It could be limiting as well though.
As far as the light output it'll be fine, but the light coverage might leave you desiring more. If you don't want to keep many corals within that extra 12" of space then you'll be fine.. you could also get away with corals that don't demand much light on the edges as well.. but keep in mind that the first 2 inches of t5 fixtures don't produces as much light as the middle.. so that will limit you even further.