256 watts is more than enough already (unless your tank is extraordinarily deep). I would actually lower the wattage if it was my tank. With such high light levels, you'll have be spot on with your CO2 and fertilization regiment for proper growth and to fend off algae. Speaking of 'spot on', yeast systems are not exactly consistent and you may have to use multiple bottles for a tank of that size.
In addition, Emperor filters are notorious for off gassing CO2 mostly because of their bio-wheels.
How long will your photoperiod be?
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29 gal of planted tank
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