There is zero truth to that statement. Stainless is a very good product for headers. The underhood temps will be a VERY little bit higher, but not high enough to make any kind of difference. I have tested quite a few different companies headers, and I prefer the Stainless ones. They are more durable and less prone to cracking. I don't buy them for the looks, I like the durability. I always wrap my headers, and the ceramic mild steel ones are more prone to cracking while wrapped. My turboed lude created more heat than a Stainless header could even think of making. So there is no problems.
