At 6V, the 6V bulb would draw about twice the current as the 12V bulb. At 12V, the 6V bulb will draw almost four times the current, corresponding to 16 times the power consumption! Since these bulbs may be most efficient when they run hottest, I conservatively estimate you will get 20 times the luminance running the 6V bulb compared to the 12V model.
The problem would be in having enough time to measure the improvement before the fuse luminesces and the reflector melts.
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7