Hi paddyb,
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If they have confirmed that this is the actual material in your mattress then the 55 kg/m3 density converts to 3.43 lbs/ft3 which as you mentioned is a little lower than I would normally consider but it may still be worth considering if you are comfortable with the possibility that you may need to replace it more frequently.
I’m looking forward to any additional updates you have the chance to share once you’ve had the chance to sleep on it for a while.
Phoenix