Whether Xibo for Windows will encounter any performance issues when installed on an unlicensed Windows 10 PC has not been tested. Please also note that Microsoft’s terms do not permit using Windows without a product key.
Windows 10 does indeed allow you to install it without a product key and it’s suggested that the effect of this is largely the loss of cosmetic options such as those you mentioned. Please consider however that this may not be the case and could change in the future.
An example of that would be Windows 8.1, which also allows you to skip activation when initially installed. Some users state that full screen messages requesting activation can appear on startup and periodically while the machine is running. Whilst it seems possible to close them, they will reappear and interrupt what is currently on screen. This would be something to consider if you intend to use an unlicensed Windows PC that will display your digital signage.
So in short, beyond the legal issues it is also quite possible that you will find performance is either affected or interrupted, so I would not recommend it.