Technically by buying a thx spatial capable Headset you have already paid extra. For example my JBL Bar 9.1 Channel Soundbar System with Surround Speakers and Dolby Atmos costs AUD$1495 and is equivalent to a 5.1 setup. A JBL Bar 5.1 Soundbar With Wireless Subwoofer costs only AUD$995 so I paid an extra AUD$500 to get Dolby Atmos. My JBL soundbar has 9.1 channels (standard 5.1 setup plus 4 upfiring drivers on the front left and right & rear left and right to get 9 channels) but Dolby Atmos is not a surround sound technology as sound does not assigned to one channel instead it is a 3D surround sound technology and sound gets assigned to a position in 3D space. If you have bought a thx spatial capable Headset then you have paid extra for all those multiple drivers in the left and right earcups compared to a normal headset.
One thing I could not work out from my quick research into thx spatial capable Headsets, if the THX Spatial Audio - Surround Sound Application comes free with the purchase of a thx spatial capable Headset or if it has to be purchased separately. Because the THX Spatial Audio - Surround Sound Application is needed to get Spatial Sound working in MSFS if you have a thx spatial capable Headset. The THX Spatial Audio - Surround Sound Application can be used “with analogue 3.5mm headsets, USB and Bluetooth headphones, anyone can upgrade their 5.1, 7.1 or stereo audio experience.” just like the Dolby Access app can but costs AUD$5.17 more than the Dolby Access app coming in at a price of AUD$27.62: THX Spatial Audio - Surround Sound Application.
While the THX Spatial Audio - Surround Sound Application does have a Microsoft Flight Simulator game profile for THX Environmental Mode:
I would still recommend using the Dolby Access app over the THX Spatial Audio - Surround Sound Application as Microsoft Flight Simulator has native (i.e. built-in) Dolby Atmos support:
^ “DOLBY ATMOS MULTI-CH PCM” is displayed on my JBL BAR 9.1 True Wireless Surround with Dolby Atmos soundbar when MSFS is running.
As a side note you can’t purchase an AV receiver with THX Spatial Audio:
While you can get Dolby Atmos audio with any headphones using the Dolby Atmos software, you can also improve the experience by selecting special multi-driver headphones. These multi-driver headphones effectively mimic what you get in a Home Theatre set up by placing audio drivers in the unit:
Dolby Atmos Headphones – Which are the best ones?
My advice is if you have purchased a thx spatial capable Headset is to use it with the Dolby Access app rather than using the THX Spatial Audio - Surround Sound Application.