I have a valve grinding set that uses stones. How difficult is it to grind out the heads to an oversized valve (say a 1.50" to a 1.60" exhaust valve).

I have only lightly grinded on heads to clean up the seat.

Thanks

i think it'd make more sense to replace the seat

It's easy. Just port out the mouth of the seat to approx 85-90% of the valve diameter, then hit it with your seat cutting stone.

Have you done this successfully?

Yes up to 1.55" because those are the valves I was using. I see no reason an extra .025" on each side would be a problem.
I usually cut a wider seat anyway to give me room for better radiusing on the edges.