Those are a full pound heavier than my actual DH wheelsets.
to put that statement into perspective a fair few factory supported world cup racers use 500-510g rims... and 630g ish is pretty much as heavy as any WC DH bike will have these days.(and you said 570g is light for downhill rims)
500g DT EX471 rim in tact
You don't need to spend anywhere near $600 to get lighter off the shelf wheels £100-200 is about the starting point if you're patient enough to wait for something discounted heavily from one of the big online stores.
Here you go. $2400 wheels and 1800 grams. Almost 700g lighter than mine for $2100 more, or $3 per gram.
[Tested] crankbrothers Synthesis E11 Carbon Wheelset
or buy something pre-built at at around that weight with a large discount online.
eg.
DT Swiss X1900 Spline 27.5 Inch Wheelset
$112 for 1729g
The World's largest electric mountain bike community.