Three angle valve seat

[mk1-mark]

Those cheap valve cutting sets are total rubbish. You are more likely to irreparably damage a valve seat with one of them than improve it. As has been said, this sort of work is best done by a professional using a centreless valve seat cutting machine.

As far as reliability & performance are concerned, 3 angle seats are always worth doing, but as with all things, if they are worth doing, they are worth doing properly!