I'm not sure whether the +% smithing bonuses stack from items, but I found a ring, necklace and gloves of smithing and did some testing.
Normal daedric 1h sword - 30 damage
Improving it to legendary without smithing bonuses - 53 damage
Improving it to legendary with smithing bonuses - 62 or 63 damage
My items are:
ring - 15% smithing
necklace - 17% smithing
gloves - 20% smithing
This tells me only 20% smithing was accounted for, so I guess they don't stack? Forgot to do tests with only gloves on...