I'm no expert on pleco's by any means, but from what I've read a lot of pleco's grow slowly because of the difference between their wild habitat and their captive one. Primarily from what I understand is that they're constant grazers and are used to living in very oxygen rich fast flowing waters. So I'm guessing if possible perhaps place something for them to graze on infront of a low mounted powerhead. The way a lot of pleco's deal with the flow is with the suction from their mouths and attaching to stones and such they're grazing on and in this way can handle flow that would over exert other fish. So I'm guessing between a slightly raised temp. some cucumber/zucchini attached infront of a powerhead you maybe able to stimulate some faster growth. Just my opinion, hope it makes sense and is helpful.