Hi pete, The wire size on your battery packs are sized for the ampacity that they are intended to deliver. That beeing said,simply replace the end. Although,if you take the time,& effort to up-grade the battery conductors it will only help, I can't imagine noticably, but in theary, yes.
Your speed 400 may already have the propper size capacitor installed within the motor case. The ones I have encountered have some means of identifying this. look close. I too have decades of rc experiance, and finally tried electrics. Brushless first ( times 13 ) and now playing with 3 brushed projects. Hope this helps, Snuts