You have more than enough light that is why there is no issue. The watts do come into play on what you can really house. He has a 60g with 4 24w bulbs with a total of 96w. Where you have 6 39w bulbs I am guessing that gives you 234w or 6 24w bulbs that give you 144 in a 40g. There are a lot of members that run T5?s but some are overdriven to accommodate there needsthere are plenty on us using T5's with SPS and LPS dominated tanks... i have a tek 6 T5 on a 40 breeder and i have no problems with it and my acro's are in the middle of the tank lol not even all the way to the top
Just my opinion
rick