I've got three 250watt Ebo Jagers laying on the bottom of my rubbermaid filter with no probs at all. All set at max (90 degrees).
I hope your not plannin in putting them in the tub without some kind of protection, cause these meat heads will take ah hankerin to them. Awhiles back, I housed my heaters in PVC tubes that I turned into swiss cheese so the heat could dissipate into the water. Didn't like the look, plus he took a whack at the wires, so now everything is in the wetdry filter next to him.