With a "test pipe" only (ie, no cat), and a modern cat back muffler system (eg Dynomax Ultraflows, which are virtually"straight thru" mufflers), you can achieve nearly the same perfomance as an open "dump pipe". Unless it was a full-out race car, you'd probably be hard-pressed to measure much difference.
Some race tracks in urbanized areas won't even allow fully open, un-muffled exhausts.