> It does, I already have a single twisted pair running in the walls of a building where it is hard to run new cables but I can't get more than 100Mb/s at the moment.

The 100mbit and gigabit variants of SPE require higher grade cabling than their multi-pair counterparts. 1000BaseT1 requires Cat6A, which would normally be needed for 10 gigabit. It is very unlikely that you have a single pair of Cat6A grade wiring available, or that anyone does anywhere.

They're not really for reusing existing wiring, they're for reducing wire count in new harnesses in automotive, aerospace, and other applications where size/lightness matters.

The 10 mbit SPE varieties on the other hand, those are more closely targeted at reusing existing wiring, but they mean the sort of wiring that might have previously carried CAN or other common control bus.