Xzyl is correct, there's two problems that arise with Mugs.
First, cyberlimbs can't hold any bioware. They cannot benefit from muscle toner, so you're effectively paying for the full package and only getting 2/5 of the benefit - meat leg and torso.
Mugs' physical attributes are Body 7, Agility 3(4), Reaction 3(5), Strength 6. This applies to his torso and his one meat leg.
His custom cyberlimbs are Body 3 Strength 5, Agility 5, Reaction 3.
His standard cyberleg has 3s in each stat.
The average of these stats are Body 4, Agility 4, Reaction 3, Strength 5.
That's not what you expected, is it?
A better way to augment Mugs is to go after muscle replacement (less nuyen more Essence), or muscle toner + muscle augmentation (more nuyen less Essence). With each option having an Availability of R*5, you can start with Rating 2 in either setup.