Is it OK to use? My mechanic for my RC30 says you can't use it on the RC30 since the stock valves have a hard coating. The lapping compound will remove it leading to accelerated valve wear and feelings of unhappiness. I'm wondering if it's the same with the NC30.

I don't think there are any issues with it - I lapped mine on the race bike using the std soft and hard gubbins and it was fine.

You are lapping the seats too remember and the benefits are pretty much worth the risk IMO

No problems here either. Just make sure you remove every single trace of it afterwards.