Hi edge,
Thank you for your question.
We could customize Windows Start screen by GPO on Windows 8.1 by the following link:
https://technet.microsoft.com/en-us/library/dn467928.aspx
Notice: Start screen control is supported in the Windows 8.1 Enterprise operating system, the Windows RT 8.1 operating system with sideloading enabled, and in all editions of the Windows Server 2012 R2 operating system. Start screen
control is not supported in the Windows 8.1 Pro operating system and the Windows 8.1 operating system.In Windows RT 8.1, you can use local policy settings, but only if you turn on the Group Policy Client service.
If there are any questions regarding this issue, please be free to let me know.
Best Regard,
Jim