In SR4, Cyberlimb Enhancements are listed as

"Cyberlimb Enhancements: All cyberlimbs come with Body, Strength, and Agility attributes of 3. These values can only be augmented by cyberlimb enhancements—enhancements from other cyber- or bioware systems have no effect."

Now does that mean that any test you do with that cyberlimb has those ratings added to it? If that's the case it seems a little...over-powered.

If you keep reading that section it explains how it works. But briefly, if its a thing only affecting that part, you just use that strength, if its something that affects a lot of parts, you average it.

The correct word is substituted.

If you use a cyberlimb for an action, you use that cyberlimbs stats, not your own stats with the cyberlimbs added.

OTOH, if you started with all 1's in physical stats, it's a free upgrade - and you can spend instead of Karma to upgrade your statistics if you're dedicated enough.