Custom Fit attribute points can only bring up your limb Attribute scores to your current meat body values, but most critically do not cost Capacity.
So if you had a Strength 6 / Agility 6 meat body, your Custom limb could also be 6/6 and still have all it's Capacity left for modifications.
You can then use the regular cyberlimb attribute increase to go higher, but only by four points as you hit the Augmented Limit.
-k