Does BT TW still have a higher drop rate for Ashes of Al’ar than Tempest Keep, even with the Collector’s Bounty buff?

Title question basically summarizes it. I got Invincible (12 tries total), and am ready to unleash my alt army on my next mount. However, given that the TW for Ulduar and BT are known to have higher drop rates for their expansions’ mounts, I was wondering - would it be wiser to run all of my alts through TW BT prior to doing Tempest Keep? Or is there at least any confirmation that the TW has a higher / lower drop rate than the current 6.70% it has in TK while the Collector’s Bounty buff is up?