Had a look inside an ep3 wheel today and it has the recess like the fn2 wheel.
I've got eibach 15mm spacers on the rears of my fn and they fit no probs, no need to change studs either.
I would check how much of the thread is used with the nut. With 10mm spacers I literally got about 1 and a half turns on the nut before it tightened. I had to buy longer studs from ARP.