These particular lights are mostly designed as accent lighting that has an omni directional bulbs that are not designed as task lighting. I have lighting at home that I've placed task lighting in because my space needs it. Luckily these bulbs are easily changed out for any standard base bulb that suits your needs.

The lumen output is 390 lumens for a 4w which is roughly equivalent to a 50w standard incandescent. These lights are pleasant when dimmed.

What is the highest lumen bulb these lights can take? e.g. 600 lumens?

These can take up to 60 total watts. However you can use LEDs to get much brighter light because they use less watts for each lumen generated. They use a standard e26 base bulb.