Solder in general:
Lead-free solder is always better than leaded solder because lead is a terrible conductor and has high resistance. Unfortunately, leaded solder flows much better, melts at a lower temperature, and gives that shiny color.
If you properly solder using lead vs. lead free solder, the lead-free will give you a much stronger joint and less resistance (lead is a soft, weak metal.) The key point is to use proper soldering on lead-free solder (higher temperature, and you must use flux.) The reason a solder joint breaks, ever, is because it was not soldered correctly in the first place (unless your car ejects your batteries 30 feet in a crash.)
Instead of spending $60 on solder, you can get a 1 lb spool of testers lead-free 96.5%tin, 3% silver, .5% copper solder for $25. The 1 lb spool should last you for at least 5 or more years. (you can get either rosin or no-clean flux in the solder.)
Ultimately, racers will do whatever they can to be faster, and unfortunately, companies exploit that. I am sure that axxis solder is great, but you can get similar results for less than 1/2 the money.
Flame suit on.