Each car reacts different to mods. It depends on a hell of a lot of factors, even down to how the car was run in from new.
10-20bhp is an estimate but is very common with an intake manifold due to the restrictiveness of the standard intake manifolds.
Best way of proof is a rolling road printout but that costs money.
It's manily trial and error with mosts mods.
A good example I can give is when I bought an exhaust manifold for mine, I heard varying stories of what they are like, some people claimed they did nothing, others claimed they did. It turned out to give my car a slight bit more pull but sounded rough as. So I took it off and sold it, ended up losing about £50 but I now have a better idea of what they offer.