The 50K hours lifespan is only for the 5mm LEDs, only if the driver is ripple-free and surge-proof, and ONLY if they're in a cool environment. Each red LED needs 2V @ 20mA = 40mW. For an input power of 300W, you'd need to wire up ~7500 of them. How much is your labor worth to you?
I've seen this thrown around before, but I'm going to need more quantitative data. I know a plant's response is almost the opposite of ours, but since the SPECTRUM of HPS and the LEDs you propose are similar, I would think the luminous efficacy values would also be similar. HPS is 130lm/W, and you're claiming for a similar spectrum that LEDs would be 400lm/W, and that's just ridiculous.
I'll ask another source I have.