when x11 came out it was true thats it was ~40% cooler than scrypt, but since then an improved miner was released and it's now more like 10-20% cooler only.

Heat wise you can lower you memory clock as like i said it does help hasrate but not but by a huge amount. Other wise its the same old lowering voltage, mine are currently running at 1.1 volts but you could still go lower than that if you have heat problems. Mine where at 0.9v i think for scrypt mining which is much more energy efficient than default voltages.

Of Course if you lower voltage then you will need to lower you core clock too to maintain stability

eg if you run at 1.00V you will probably need to set your core clock in the 880-980 MHz range, you will lose performance but running at 90C is not cool.