550's are usually ok with a 16g, but 400whp your really going to be squeezing everything out of that turbo and going to need the extra fuel.
what kind of A/F gauge?? I'm assuming you mean a wideband, if thats the case I would think about swapping the EGT gauge out with maybe fuel pressure gauge.
Alcohol injection will allow you to squeeze alittle more power out of that setup.