As I said - I don't use their pistons. But it's true most manufacturers son't warrant race parts. I've generally found a lot can be done with them if you approach them in the right manner. Simply demanding warranty on a failed race part probably won't get you anywhere. But supply them with a forensic analysis of what happened to their parts in service and many will offer advice and help. At the end of the day they know people soon talk about parts that fail on a regular basis. If Wiseco's recommended clearances resulted in a lot of seized pistons you can bet the Internet would soon be full of "Their product is Sh*t stories". Many years ago it was all we used in the 900 Kawasaki's we ran. Never had problems with their big bore kits. Cheaper and better than stock parts. We used their specs from memory.
But if you feel that you can second guess the manufacturers and do better - by all means, do so.