Hi daniel199260,
Thank you for your enquiry.
"I know a lot of people run 12mm plenum spacer and 25mm manifold insulator, but have not seen or read of anyone running 25mm top and bottom? Wondering what the go is there. Is it do able or?"
The reason is that for NA (naturally aspirated) applications, ideally you would want to run a 25mm manifold insulator and 12mm plenum spacer. Otherwise, if you're running some form of boost, we would recommend a 25mm plenum spacer and 12mm manifold Insulator as the thicker plenum spacer offers even greater air distribution between cylinders when dealing with greater air flow. On this note, if you attempt to run a 25mm plenum on a NA ecotec, it will impair driveability and performance.
If you have any other questions, please do not hesitate to ask.
Kind Regards,
Michael