If this is "unified memory", the GPU will eat up at least half of that. So that leaves you with 4. To run PS and Lr at the same time comfortably, you should have 32 GB. 16 will probably work, but that's pushing it.
I read about unified memory and it seems to me that unified can speed up a bit process and reduce amount of memory needed. Otherwise I do not understand where is real advantage, please explain or give me link. What I understand is that Windows without unified memory is copying computing result from RAM to GPU memory thus waste time on that task. My system has 32GB RAM plus 6GB GPU memory. Someone has 32+12 or even 24 so total memory on Windows computers when 32GB RAM is installed is usually (when dedicated GPU is present) more then that. My conclusion is that Apple is speeding up things by using same memory for CPU and GPU and reducing amount of memory needed but... you can not change amount of RAM nor replace GPU.