Hello. The carbon issue is much more common in the United States than in the UK. Quite possibly because we have better quality fuel over here.
Hardly any members on this forum have reported a carbon issue but than again the 250 is quite rare here in the UK compared to the United States .
I've never had any dashboard rattles in the 250 but did have a rather annoying rattle in the IS220D.
The Lexus IS250 is a superb car. If you find one for sale, buy it. You won't be disappointed.