UltraBrakes tips

I tuned return springs to the max but seems not enought.

With proper setup the BetaC butted spring leafs (not just simple Titanium wire!) give a correct preload range on brake arms.
Best breaking performance should be achieved with low cable friction operation, low spring preload, low brake arm flex and well clean braking surfaces. 
Naturally on a hi-end bike cable lines are expected to be correctly made. Please note that housing lenght and housing routing are even more important than liner and cable quality.
It would be silly to reduce stiction employing 4x 2RS cartridge bearings and tricky spring sliders on expensive brakes and then couple them with a poorly arranged and un-smooth cable routing.

Let's see frame/rim standards:
A) measure canti boss distance on frame/fork: should be 80-82mm
B) check rim width, MTB standard 23mm
C) diff. value (A-B):  58mm average (this is the measue that influences linear spring position)
With sensibly higher diff. value you may loose spring preload, then use "thick pad spacers" and eventually move pin to upper boss hole.
With a sensibly lower diff. value use "thin pad spacers" and reduce spring preload.
Additionally note that boss holes are not always well centered at frame welding/glueing. As example Scott scale frames sport countless errors: on brake bosses, brake bosses holes, cable routing,  cable stops, bb shell etc.