I don't believe anyone is getting into a sleeping bag at 43 C unless they were trying to imitate being in a sweat lodge.
But yeah, Doug64 is on the money in that you sleep on top of your bag in warmer weather or to the side of it so when it cools in the wee hours you don't even have to wake up properly to grab your bag and pull it over you.
For those types of temps, I'd happily recommend a bag similar to what I currently use.
Down rated at 800+ and about 450grams of it. Goose down mixture of 90/10 with the ten being feather to increase loft.
Lower temp rating being:
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[TD="class: data"]Comfort 2ºC / Limit -4ºC / Extreme -21ºC
Total weight approx 820 grams.
My bag is:
Endurance Sleeping Bag Express 400 XL | Down | Sleeping Bags | Equipment | Macpac Australia
They have regular sales where I think from memory I paid $350.00 Au which is around 50% off normal retail.
I'm not saying you have to buy the same brand bag.
What I'm trying to suggest is that a quality brand with the same or very similar fill will cater to what you've suggested your needs/temps are.
Saying that Macpac is a NZ brand that have an international reputation for quality and durability for their outdoor gear including tents, packs and sleeping bags.
Top Notch.
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