Hi and welcome.
It is usual for exhaust valves/seats to deteriorate first as get much hotter.
General rule is that if you cant get rid of pits by lapping with coarse paste, will need to re cut seats.
You can try using an electric dril on stem to rotate, but keep lifting valve up and only do for very short period, say 10 secs.
Finish off with fine pasted by hand and make sure you remove all paste.
Wide seats will effect performance, but IMHO unlikely to notice on a 400