Is it allowed

Try reposting this in help and support. From what I know it should be fine, WTFast was promoted in the community in the past for increasing connection, but I would double check from a moderator (or just send a support ticket with the question).

Yes, it should be fine. All it's doing is freeing up processing power on your machine by stopping non-critical processes. This is theoretically something you could do manually, as well (and it would generally help with overall PC performance, not just in-game). It doesn't do anything to the game data or interact with the game client at all.

The gamebooster does things you could do yourself to speed up your PC such as disabling certain services, changing performance mode to high performance, favoring performance over visuals for your desktop, etc. It's allowed.