Tonkie, having chased the illusive improvement of brake feel/bite/power on classic bikes, I know from experience there's not a lot you can do. Yes, if you want to change calipers, discs, lines, pads, m/c's etc then you will eventually find something that works for you.
But on your STrom I take it you just want a bit more bite, changing the lines will give you little or nothing because your lines are pretty new anyway. Get it booked in and have the caliper cleaned and the recommended pads fitted, but only if you can't live with it.
My 650 is pretty weak on the rear but in conjunction with the front it stops just fine. Although if I was changing the rear pads or just cleaning the caliper I would probably change to the HH. After around 5,500 miles my pads are still like new and the discs don't look like they've ever been squashed by pads, this is probably the problem, perhaps the HH would help.
Hope you work it out.
Upt'North.