I helped build a setup for a square tent, 4'x4'. He used 3 of those 4.6" bars /3'long. 3x 3070 ea running at 1.4a with a single fan in the centre of each bar (so very similar). This gave a good coverage, negligable heat but the bars were expensive.. overkill (on h/s and wattage) perhaps. I'm really not sure about the narrower lengths for higher currents, personally i'd build one bar and experiment it's easy to buy/ tedious to sell lighting equipment. The formula's for working out h/s surface area mentioned on here are pretty conservative to keep cob temps well down even passively. Maybe you should ask that specific question (for help on choosing heatsink) separately or in the H/S thread if needed, so many alternatives it's hard to make suggestions without taking the whole grow into mind/ suggesting every part to go with a 'system'. You are right to spread the light as much as possible and low current /many cobs helps.