The xmp setting is funny… I also heard some funny things/settings about pre-build PC’s.
The OP can easy check that with tools like CPU-Z ( not all users are familiar with bios settings ).
I personaly would feel also better with 1000w , simple more headroom, but a good 850w should normaly works fine ( its also within the recommendations ) and if he get no longer issues, why upgrade… Of course there are many additional factors, like OC version of a gpu, etc… ( also the 3090 needs also a little bit more power ). I am excited how its end for the OP 