One thing I get from this forum is anything over 200W on a 120 gal tank is insane lighting and you will grow nothing but algae.
But everything in my own experience makes me disagree. I have an 8 bulb Aqua Medic 6 inches above the water on a 24" deep tank and I run 4 bulbs for 2 hours, all 8 for 5 hours (read 424 watts of T5), and then 4 for two hours and the fixture goes off.
Yes I use pressurized co2 that is controlled with a Milwaukee PH controller and I pump as much in there as possible without killing the fish. And yes, I dose ferts daily.
The only algae I ever get is from hobbyist plants I buy that have it when they arrive. New growth never gets it and I have some pretty stunning weed that grows like mad... but that is the goal right?
When I get weed from the pros in the forum that are using 2-3 wpg the plants are sometimes a bit pale. Within a week in mine they look a nice healthy green and the red plants get good and dark (keep the iron up) and all is good in the aqua farm.
You can have 2wpg of t5 and grow algae like mad... lighting depends on how you fertilize and how you administer the co2 and how you get oxygen back in the tank when it is dark.
I am sure things in the form will not change and everyone will still consider 2wpg as high light.. but in my experience.. It is moderately high.
4wpg of T5 on a high tech tank is definitely high light, 2wpg isn't in my opinion, not even close.