Depends on the thickness and the material of the backing..
Do you mean strength against failure or do you mean adding draw weight?
As a general example, if you have a limb 1/2" thick and you add 1/16 thickness, the limb will be twice as strong.
If you put on a thin fabric backing, maybe only a little difference, but if you add bamboo or hickory, you will almost certainly have to retiller.
Also, be sure the backing is appropriate for the belly material. Clos th works with many things because it is thin and does not move the neutral plane much, but bamboo is so strong it can overpower most weaker woods.