Here is a list of everything tested in the process. I think the most intriguing improvement to come out of all this is finding that
Uncanny Pocketwatch is attainable on a 10/11/20 if
Adventure mode is unlocked. It does not stack, so it was worthless for the challenge but definitely something that can speed up TBC (and dungeons in general) farming.
Race:
Darkflight (Worgen) - 40% on use
Embrace of Gonk (Zandalari Troll) - 5%
Quickness (Night Elf) - 2%
Elusiveness (Night Elf) - 5% in stealth
Dungeon Delver (Dark Iron Dwarf) - 4% indoor
Druid:
Cat Form - 30 %
Travel Form - 60 % @ lvl 20 / 40 % @ lvl 10
Feline Swiftness - 15 %
Dash - 60 % *
only works Cat Form
Items:
Speed Tert Cap - 49% (75 @ lvl 11 / 111 @ lvl 20 / 171 @ lvl 29)
The Postmaster Set Bonus - 5%
Tailwind Regalia Set Bonus - 5% (cloth only)
Uncanny Pocketwatch (adventure mode req) - 30% on use, doesn't stack
Water's Beating Heart (adventure mode req) - 10% on dispel, doesn't stack (even with Minor Speed)
Rocket Boots Xtreme (lvl 15 req, engi req) - Tert speed value (not %), doesn't stack
Goblin / Gnomish Rocket Boots (engi req) - Tert speed value (not %), doesn't stack
FIgurine: Golden Hare (lvl 15 req + JC req) - passive doesn't work at all, 30% on use, doesn't stack
Figurine: Ruby Hare (lvl 15 req + JC req) - 30% on use, doesn't stack
Highlander's Leather Boots - doesn't work at all
Explorer's Walking Stick (lvl 25 req) - doesn't work at all
Perfect Dreamgrove Blossom - 20%, Druid only, Broken Isles only
Thas'dorah /
Windrunning - 25%, Hunter and the whole group, Broken Isles only, doesn't stack
Man'ari Training Amulet - 40% on use, Requires high-level char to get, doesn't stack
Enchants:
Windwalk - 10%, can double proc (stacks) using weapon swap
Gift of X - 2%
Minor Speed - 10%
Any Stat + Speed - 10%, doesn't stack with ANYTHING
Nitro Boots (engi req) - 150% on use
Gems:
Straddling Jewel Doublet - 13 speed per
SL gem
Straddling Viridium - ? speed
Straddling Sage Agate - ? speed
Fleet Shadowspirit Diamond (and all similar) - doesn't work at all
Consumables:
Gun Shoes - 200%, doesn't stack
Swiftness Potion - 50%, doesn't stack
Oddly-Shaped Horn - 410%, doesn't stack with ANYTHING
Cracked Radinax Control Gem - 429% (why?), doesn't stack with ANYTHING
Group Buffs:
Angelic Feather - 40%, doesn't stack
Tiger's Lust - 70%, doesn't stack
To Be Tested:
Drums of Speed - 15%, was not stacking before the last patch, have not tested it since
Breath of the Plains - 5%, not sure if possible to get
Swiftness - 50%, proc was not seen after multiple charges used