Sorry cant help with that, and not many here has your setup i think (us centrist). And controllers are often extremely different, and today tech is moving fast.
Again you might be right.
But i still think, the problem could be that the controller is 13A peak, halt that nominal, at some point the power is just not enough. The controller is just not able to output any more juice, because it is not designed for it.
A motor might want to reach its max rpm, but cant, because current is too low.
As the other poster wrote, a 20a controller and 48v battery is probably what you need, generic controllers are dirt cheap on ebay, and since you are an engineer, that should be no problem
you are probably interested in keeping the setup nice and clean, so a bottle battery the ones that look like a drinking bottle maybe, with a 14.4v 10000 mah lipo inside/somewhere in series is an option. (some might say that's stupid YMMV)
http://hobbyking.com/hobbyking/store/__ ... _Pack.html
that will give you 50v, then mix it with a 48v 18-20amp controller YOU select, and know what it can do and how to control it.
Or get a new 48v 10ah lithium with bms, then run it on an 36v controller, so you can use the build in battery as a range extender.
The sky is the limit.
no trolling, just freindly advice.
Id love to go to stockholm, maybe i cant get you people to give us back Skaane, halland and bleking. hehe