Well we'd already stretch our 370 LEBs about twice as far just by switching on LZMA:

@ cat < rootfs.tar | wc -c
61081600
@ lz4 < rootfs.tar | wc -c 
37505088
@ xz < rootfs.tar | wc -c
19323784 
@ ls -lh rootfs.erofs
-rw-r--r-- 1 cfsworks cfsworks 38M Aug 13 12:16 rootfs.erofs