A 24db reduction is a 99% reduction in sound. Do you really need more than that? Subtract 7 db because most people don't install them correctly and your left with 17db which is about 97% sound reduction. Riding in silence sounds rather dangerous to me.
Edit: or another way to look at it, There won't be that much of a gain going to a 31NRR rated plug as opposed to a 24NRR rated plug.
I ride every day, and in my line of work, having good hearing is critical... so I'm keen on protecting my hearing.
With the Hearo Extreme, I notice a huge difference in the sound level as compared with other foam ear plus. I can't say definitively if it's the design properties of the foam or the fact that the Hearo Extremes are physically larger than other plugs, but they are the only ear plug I've ever used that gave what I consider to be acceptable reduction in sound.
YMMV.