No you are wrong, the spectrum doesn't have nearly as much to do with it as how many LUMEN'S of light your plant gets. LEDs are expensive to use because they put out very little lumen's. Per watt they are efficient but you need ALOT of them to generate enough lumen's to grow well.its not brightness that counts the most.....its what color spectrum it produces
Its all about the lumen's, you can use a 3700k light that puts out 100,000 lumen's, vs a 7000k light that puts out 50,000 lumen's and the bigger light will grow bigger plants period.
Metal Halide and High Pressure Sodium are are the most feasible, least expensive, lumen cranking, efficient lumen per watt lights available. I highly recommend these lights over any other. Search Google for a nearby hydroponics store. I did, found one 20 blocks away...