Ive experienced the same as Padders with both the Nimbus 13 and 14, I even got the 14 2E because I though they might be too tight around the ball of my foot.
Had the same after about 3 - 4 miles severe burning on balls of feet. Now it's mostly gone. I have noticed I'm getting blisters on the side of the ball on both feet though. This might just be down to increased mileage though not sure yet.
Incidently I purchased a pair of cheap Asics Enduro 7 for £30 mainly for running off road / in the mud and I seemed to feel more comfortable in them than the £120 Nimbus even when road running.