What time of the day are you travelling? I've found that the FCC is its usual zoo-like self in the mornings with the goons before the screening, but speeds up a bit from midday until the evening rush. In this latter period I don't think you'd gain much by taking the HEX, particularly if you miss a departing train. I don't think you'll be able to use the fasttrack as it's signed for Club and First passengers (not Gold). But as with all things LHR, YMMV.

Wel thuis!