From what understand, if you don't put your engine under a lot of heavy load, like pulling long grades with your foot in it, you can get away without the hardened seats for some time.
Tetraethyl lead was used as an anti-knock additive, but also served the purpose of acting as a cushioning agent on the valves / seats.
If you run gonads-to-the-wall without leaded gas and hardened seats, you will erode the valve heads and eventually "suck" a valve (or two).....
