is there a cat back system that looks and sound pretty close to oem but could flow enough for around 500 hp?

any supra exhaust (including stock) can flow enough for 500hp. the tanabe hyper medallion and trd look stock ish and arent too loud and flow well.

so a downpipe would be enough?

yes just adding the dp will significantly improve exhaust flow. the stock exhaust system isnt all that restrictive. note that the dp will make the exhaust note louder/deeper.