Well Koni don't make Springs....they get them in from Vogtland. Last time I fitted any, the car sat pissed.
Personally I'd go Eibachs. They're a good spring and give a nice balance of handling and ride. And I doubt you're going to notice 5mm between them personally.
If you're dropping 30odd mm at the front, you'll have to get the back down 30mm at the back too, so you'll have to withdraw the bars to lower it, but you'll retain the 15/20mm height advantage over the front, which is what you want if you like going round bends quickly.