Will I gain much from going from version 7 to version 9
If you are upgrading the virtual hardware version to 9 you will get the benefits which are avilable with vSphere 5.1 which are specific fro Virtual Hardware version 9 like Up to 64 vCPU per VM, 1 Tb of RAM, Improved 3D Graphics Suppor, tNVIDIA Quadro 4000/5000/6000 and NVIDIA Tesla M2070Q
There is no need to perform Re-IP after hardware upgrade