1000BASE-T uses 4 pairs and each pair is simultaneously used in both directions. 250Mbps per pair, 1Gbps total, full duplex. You could trivially chop the 1000B

1000BASE-T uses 4 pairs and each pair is simultaneously used in both directions. 250Mbps per pair, 1Gbps total, full duplex. You could trivially chop the 1000BASE-T standard down to 1 pair and get 250Mbps full duplex. That would be similar to what SPE does. Everything you just described applies to bog standard 1000BASE-T too - PAM, echo cancellation, error correction, scrambling, etc.