This is how I understood it as well, the 4S has two SGX540s in a dual core configuration.
Also, the SGX543MP2 in the 4S is only clocked at 200MHz.
This brings up the age-old question. Would you rather have a single chip at 400MHz, or dual chips at 200MHz? While it's 384 instead of 400, I believe the same logic applies.
I would choose the 400MHz, assuming the architecture scales with clock speed.
Well, what we'd truly need to know is the amount of power it takes to run an SGX543MP2 at 200 MHz vs an SGX540 at 384 MHz.
What I don't know is how the SGX543MP2 is able to perform about twice as well as the SGX540 at half the clock speed and subject to the same bandwidth constraints. It may be that the SGX543MP2 is able to process a tile (remember that the SGX5xx are TBDRs, not IMRs like Qualcomm and NVIDIA's GPUs) with each GPU simultaneously while accessing separate memory memory channels to alleviate bandwidth issues.
But this is just some guesswork on my part.