Does 800xA Support Hard disk Encryption?
It is easier to assume what is not explicitly mentioned in product guides, sales material and users guides is not tested, nor supported.
The 3BUA000500-xxx System 800xA - Third Party Software Versions document lists what third party software is tested for each version of 800xA. The document is tagged Premium, hence you will need a Sentinel subscription and account (or ABB intranet access) to download it from the link I supplied.
You may argue that BitLocker is part of the Microsoft Windows operating system, however, no official tests has been made with it.
Modern hand disk encryption technology is not very CPU intensive but will hardly make any disk faster... the impact may be hard to predict and vary depending on computer role and software blend in use (an operator workplace typically accesses many small files and run on a SSD while application servers may work with larger files and predominantly mechanical disks).