I've just done the same thing and have 4 of the Grobeam 600's over a 100CM tank. I'm currently running all 4 at 40% having upped them from around 15%. Only when increasing the lighting have i seen growth start to happen.
If you read my journal, you'll see the problems I have been having!
Just had a read through your journal, very interesting as I have recently reached a similar conclusion. I'm using a single AquaRay 1500 Tile on a TMC 60x45x45, suspended 30cm above the water surface and everyone said they are so bright you need to have it really low intensity, but I basically had dormant plants apart from the odd low light plant for months last year after initial setup, plus lots of plants just died. I eventually increased to 60% for 7 hours a day, but with the tile being maybe 70 cm from the substrate and also a plexiglass cover to stop jumpers which gets covered in condensation and therefore diffuses the light further, I was still not convinced I had enough light. My Ech. Reni would put out a new leaf every few weeks and looked healthy, but was a dwarf, never getting more than about 8-10 cm in height. Mick.dk commented and said not enough light and that was at 60% for 7 hours.
It was around this time that I decided to put in some new plants, change things around a little, but then screwed up and damaged my CO2 solenoid. I bought a replacement from CO2 Art, but then had months of issues with equipment over Christmas and Near Year and only got CO2 up and running again last week when they replaced my atomiser as that was leaking. I ran the tank on low light for a couple of months throughout this, 4 hours a day at 30% in the hope I could keep as much alive as possible. I lost some plants, kept some alive in a fairly dormant state. I'm now running at 6 hours at 60%, lime green DC throughout the photoperiod, everything stable, but no noticeable growth yet, but it has only been just under a week. I'm planning a total rescape at the end of February, so intend to push the light levels and see if I can find the sweetspot where I get growth without algae, so if I do get algae, I can sort it in the rescape and start fresh with a better idea of the light levels required.
Just to add, I do use a ramp up and down, but only 15 minutes either end so it's less of a shock for the fish, but I am getting full power for most of my photoperiod.