For anyone that cares I got some info about sizeing of downpipes guy on YB says that 120% of the exh. wheel exducer is the rule of thumb for DP sizing

So If you have the 87/96 wheel on the S475's thats 3.4". So 3.5" off each turbo is fine.

If you have the smaller 74/83 wheel S475's then thats 2.9"... so 3" is fine.

That's assuming no sharp bends and a reasonable length of pipe. For most of the short dump style turbo exhausts 120% is plenty, no need for larger.