I have just tested the sleep modes from the examples supplied with the lgt8 core, and all I can say is it's pretty disappointing
Even in PM_POFFS1 mode which is the lowest power mode possible (only wakes up with external interrupt) it consumes over 80µA.
There must be some settings that can be improved to get closer to what the datasheet seems to claim, but it's pretty hard to guess how we are supposed to do that with a chinese language datasheet...
EDIT: when I force pins to OUTPUT + LOW the power consumption drops significantly, it's below 20µA now. If someone has some knowledge in Chinese to find out if/how we can disable ADC/SPI/... that could get interesting.
But without proper sleep functions the Xi loses most of it's interest as battery powered sensor (low minimal voltage, no regulator & power led to remove etc). Only interest at the moment would be the 2 DACs that the atmega doesn't have, but that's of very marginal interest for MySensors. It doesn't even have a cost advantage as it's only 0.30$ cheaper that pro mini but with shipping fee.