I also purchased two SEIO M2600 powerheads. The power cords are definately too short. Heat transfer is slightly noticable, but only a couple of degrees increase. I am also a bit leary about the suction cups when mounted on side glass panels as they can fall quite easily, however, they do come with several fasteners and can be mounted more securely. I did have a problem with the impeller assembly on both of them. I had them on a 12 hour timer cycle and they kept grinding at startup until they each crapped out. TAAM admitted that they were experiencing failures and replaced both impeller assemblies...no questions asked. They also recommend not to run them on wavemakers as this can intensify the impeller failure. Overall, I find them a great powerhead for their 80 Watt power consumption and purchase price.
That was until I discovered and purchased an EcoTech Marine powerhead.
Their flow and reduced heat transfer are exceptional, but the retail price is quite steep. They too have manufacturing problems that they are working out successfully.