Bamboo is highly heat resistant and can be found in 3"... Hmm...
Did he give you any justification as to why it would damage the engine?
I've had 4 3800 Grand Prixs and all have had a cone intake of some sort, with no issues.
If you're worried about it, go for the K&N. It doesn't void factory warranties on vehicles that have them and it looks almost stock. Just remember to clean and oil it regularly.