I wouldn't agree with soldering. Aircraft looms don't solder, F1, and World rally cars don't solder etc. A good crimp is stronger ad just as conductive and will last longer. Heating the copper anneals it so it goes soft right next to the stiff bit where the solder has run between the strands, this will then wobble about and work harden, go brittle and then snap off. Seen it a number of times.
The headlighs are already running through a relay btw ad the thin wall cable of the std loom will e up to the job its just it will be running right on its max rather than having a cushion in hand.
That said I'm not a big fan of high power uprated bulbs, much better are the modern high efficency ones from the like of PIAA, Osram and Phillips. They draw the same current (THERE IS NO SUCH TERM AS AMPERAGE!!!!!!! it is pseudo science) but provide up to 100% more light than standard bulbs.
Most uprated bulbs ae cheap chinese etc imports, even a 0.5mm change in the location of the element will upset the beam pattern. My 2 penneth is buy original equipment bulbs, not OE spec (everyone claims that) - only Phillips, Osram and GE supply the manufacturers.