Scaling mounts?!

Hello fellow players, dear friends.

I have a simple question which needs a definite answer. If I happen to buy a 310% speed mount (merciless nether drake) and after that I decide to buy Headless Horseman's mount - how fast will the horse go, 280% or 310% ?
- for Lordaeron (wotlk)

Could you be so kind please and answer this question for me? I've tried google, but there are several different informations and I'm left again without a proper answer.

Just checked it in game and it's 280%. Some other mounts scale to 310% when you have a 310% mount but this one doesn't.

Thank you for the info. Do you happen to know whether the same applies to Celestial Steed mount and the Big Love rocket?

Both of them scale to 310% if you have another 310% mount.

They don't scale. The 280% speed mount stays at 280% regardless how many 310% speed mounts you have.