Since you have a Golf, do the 5th brake light mod and get even more light.
Of course, a lot of people (including myself) use 1156 as the alternative. There is another alternative, 3497.
Life: 150 hours
Life: 1200 hours
Life: 600 hours
1156 are dimmer then P21W, but have substancial more life
3497 are brighter than P21W, 4x more life, but costs me $9 for a twin pack.
If you're assuming everything is linear... bump up the voltage to by .8, P21W will still not as bright (estimated 494 lumens) and will have even less life
Of course, if you ask the beloved Daniel Stern what he recommends for MKIV tail lights..... He'll tell you:
Brake & Turn = go with 3497 instead of P21W
Reverse = 796 (which is a 35W Halogen) instead of P21W