Do I need SPI firewall on my modem router?

[Bill W]

Like 99% of everyone out there, I use a modem/router to access the internet. I currently have SPI firewall enabled by default on my modem. About 6 months ago I disabled it for a week or so to see if it would make any difference speed wise, but couldn't see any discernible difference for better or for worse.
My question is: Does enabling SPI on my modem offer any benefits over NAT without firewall? I'm happy to disable SPI if NAT does everything that it should.

Thanks