You do not get any bonus exp. for sweeping a horde in one move.
After testing this further, I have found that you do not get any bonus experience for sweeping a horde of wild pokémon.
I used two pokémon to test this; a locally traded (1.5x exp.) Venusaur, and my own Hippopotas.
n.b. - I fainted all five opposing pokémon in one move.
Test 1: 5x Level 23 Durants, Terminus Cave. Traded Pokémon
What I would normally get for 5 single Durants:
169 (Durant base exp.) x 23 (level) x 1 (no lucky egg) x 1.5 (traded) x 1 (wild Pokemon) / 7 x 5 (no. of Pokemon)
169x23x1x1.5x1/7x5 = ~4165 exp.
What I received:
4160 exp. **No Bonus.**
Test 2: 4x Level 8 Zangoose + 1x Level 8 Seviper, Route 8. Traded Pokémon
What I would normally get for 4 single Zangoose + 1 Seviper:
160 (Zangoose/Seviper base exp.) x 8 (level) x 1 (no lucky egg) x 1.5 (traded) x 1 (wild Pokemon) / 7 x 5 (no. of Pokemon)
160x8x1x1.5x1/7x5 = ~1371 exp.
What I received:
1365 exp. **No Bonus.**
Test 3: 4x Level 8 Zangoose + 1x Level 8 Seviper, Route 8. My Own Pokémon
What I would normally get for 4 single Zangoose + 1 Seviper:
160 (Zangoose/Seviper base exp.) x 8 (level) x 1 (no lucky egg) x 1 (not traded) x 1 (wild Pokemon) / 7 x 5 (no. of Pokemon)
160x8x1x1x1/7x5 = ~915 exp.
What I received:
910 exp. **No Bonus.**
If anything, what this little experiment has uncovered is that Gamefreak's rounding system is a little odd, or they purposely decrease the amount of experience by approximately 5 points for some reason. Either way...
...no bonus experience.
Sources:
List of pokémon base experience yields.
Experience gain formula
Edited for formatting.