I don´t use WTFast but something similar (a PPTP VPN Tunnel to a Server at another ISP).

I use it when my ISPs Uplinks at the VIX are overloaded due to netflix and stuff. That´s the perfect scenario where WTFast really may make a difference.

A VPN Tool can improve in 2 ways, you can sometimes avoid congested Links (depending on where WTFast/VPN sets it´s endpoint) and/or avoid P2P Traffic Throttling, due to encrypting the Traffic (and ISPs see VPN Traffic more important than your game traffic)

But be aware, results may vary from user to user and from 0 to hero...

for example, if your "last mile" is broken, you wont notice anything.