This question comes up ALL THE TIME. The books simply weren't written with a full body overhaul in mind. All the math makes sense if you take a fleshy guy and replace an arm here or a leg there. But when you replace every bit of squishy, there's no sense in stat'ing yourself the same way anymore. Call it what it is, a drone body with grey matter upstairs.
According to RAW (which again, loses focus when you go for a full body replacement)... you still have to average everything if the check involves your whole body instead of just a single appendage. So yes, 9's all the way around, but somewhere in what little squish is left is a 2, it's going to bring your average down a bit. Especially with your example, since you're only using Lower Arms and Lower Legs, which means there's still flesh and bone connecting them to your torso. Plus the Torso is just a shell, not a full replacement like the rest.
However, there is talk that maybe the Rigger book or something, will have details on Jarheads.