well i dont have any definite answers for you since i dont use slic3r and i have not printed that brand of filament. i have printed a boat load of ninjaflex though. enough that i have one printer just dedicated to it that runs about 12hrs a day. for the ninjaflex i run that at 250deg. hotter is def better. i makes it thinner so it extrudes better. i would turn the multiplier up, temp up and the speed down a little for the perimeters. i also found retraction to be almost useless for ninjaflex. it just stretches it like a rubber band. i found it causes problems but i also never tried slower retractions like your doing