Well. Think of this way, if you use Storm Gust as an example, 5% of 5 seconds is .25 seconds, which means you'd be using SG a quarter of a second faster. Two rings would be twice that.
I wouldn't suggest using Expert Rings over a Celeb ring or Bris unless you put Stings in them. Even then, I wouldn't say that being able to use it 10% faster would be worth losing 10 intelligence, which would result in a noticeable weaker spells.