How much heat will it actually repel and will it even seal properly

https://www.ebay.co.uk/itm/SUBARU-IM...8AAOSw011Z0SEU

 

Things like this on their own usually have little real effect. But teamed up with inlet thermal spacers, heat reflective tape and exhaust wrap, they all add up.
If your replacing the gasket anyway it's only about a tenner so probably worth it.. but remember if it's only a tenner how much can it really do.
Should technically help lower intake temps and should increase throttle response. Just don't expect any real world differences