Drysc from Blizzard comments a bit on how the Riding Crop Trinket Stacking works with new items...
There's not any current intention to allow either on-foot or mounted travel to be increased by items beyond 10% that each type of travel already allows. So regardless of what you have equipped you're not going to get above 10%. The riding crop is really just a one stop replacement so you can clear out some space you might have been using for other speed increases. Hey, there's the bag space solution you've been wanting! Enjoy.
Thanks