6500K is the colour temperature, what they call cool white. 2700K is the other end, warm (yellow) white.
10W LED bulb would be around 1,000 lumens.
4 ft conventional fluorescent tube is around ~2,000 lumens.
That said, even if you do get a matching 20W / 2,000 lumen LED bulb, in my experience a longer 4 ft LED tube is better at spreading light.