i stepped up to 4" for a few reasons.
1st being the TB is 90
2nd being by stepping up to 4" it allows the compressed air to cool some since its now taking up such a larger space. [same theory as exhaust sizing. to large of pipe and you lose youre heat and it slows down the exhaust pulses but it works opposite for boost. You want lower IATs, so if you can slow the air down by increasing the piping size you then lower the IATs since that compressed air now expands into the larger area and cools. the pressure loss you from increasing pipe size isnt a concern when doing with large compressors like what V8 singles run]
3rd the 4" pipe just looks much better IMHO
4th its what the silverado had