At 550hp you will notice virtually nothing, may free up a mininimal amount of hp but that’s it.
Your turbo will be more restrictive than the exhaust at that power.
Having said that if you need a downpipe made anyway then I personally would go for a 3.5 inch downpipe then leave the rest of the system at the normal 3 inch diameter.
That way if you do increase the power then you already have a bigger downpipe fitted

Cheers Paul