burned valves, what exactly does a burned valve look like?
I have seen a few valve calamaties in my short time as a know nothing and burning exhaust valves is one of those conditions I might consider mythical.
Certainly changing your exhaust tuning could effect the temperature of the ex valve. Determining the actual point at which it is burning could be a bit more subjective.
If your pipe creates more HP I would safely bet it is not secretly destroying your exhaust valves, or their seats.
The seal of these parts is part of the true HP equation, IMHO.