I'm not sure, but i think its because t-5's are more efficient than CFL. So if you are using 2 wpg of t-5 then you are starting to push the low light boundary. Where as 2 wpg of CFL will still be considered low light. Someone correct me if i'm wrong.
Sorry but an 8" pelco will destroy any planted tank with anything other then anubias and maybe some types of ferns/ moss. not by eating but just his size will constantly uproot the plants.