Get at least 2μmol per W from a 300W LED! Grab a GN Telos 0010.
Like all Grownorthern Telos LED lights, this is one of the most efficient LEDs you can get in the UK.
It's effective for roughly 85,000 hours - that's about 10 years! You're not tied into today's technology - you can upgrade your system as CREE release new LED modules.
They don't cost much to run, either - it's only 300W!