FWIW, I don’t use Particle firmware but my own testing on the Xenon hardware with CPU off and one RTC running from the 32 KiHz crystal shows about 35 uA running from LiPo at 3.96V (measured with a uCurrent; numbers are very sensitive to supply voltage).
It drops to about 27 uA if the MX25L32 is placed into deep power down mode before going to sleep.
Powering from 3V3 directly with 3.0 V with XC9258 disabled by shorting CEn to ground I get about 22 uA (this configuration also removes power from the MX25L32). Anything lower than that would require mucking with solder bridges which is beyond me.
I’m pretty happy with 27 uA; my sensor apps spend about 99.92% of their time in sleep so that’s years of service life out of a decent sized LiPo.