The default optimization level for the PTXAS component of the CUDA compiler (the PTX-to-SASS compilation) is the same as -Xptxas -O3, so you don’t need to set anything. One would typically lower the optimization setting via this flag, e.g. -Xptxas -O2.
All -Xopencc settings only apply to the Open64 component of the CUDA compiler (for C++ -to-PTX compilation). Already several years ago, Open64 was relegated to the role of legacy compiler for sm_1x architectures only. With CUDA 7.0, sm_1x support and the Open64 compiler were removed entirely.
The CUDA compiler now uses NVVM for C++ -to-PTX compilation. NVVM is derived from LLVM, a modern compiler infrastructure. I am not aware of any -O optimization switches one can pass to NVVM, by default it applies full optimization.
To set the optimization level for the host compiler, one can use -Xcompiler, however the Visual C/C++ compiler does not seem to have an optimization level /O3 (at least not in MSVS 2010). I does have /Ox, though, so -Xcompiler /Ox is accepted.