I've done my rear lenses, but in the end I had to settle for a tint a little lighter that I wanted. In any event, you can't do them too dark, as they will fail MOT. You will also need a separate set of red reflectors (unless you have them already fitted to the bumper).
I tried putting in bulbs of different wattage to those fitted, and whilst they worked, I immediately got the warning indicator on my instrument cluster (the orange 'lightbulb' warning led). Seeing that orange led really pi**ed me off (even though I knew what it was for). I suppose I could have covered it with black tape, but I preferred to go the 'normal bulb' and lighter tint rule.