As you can see in the photos below, the Reatta has a restriction in the stock exhaust manifold. Removing it should give more flow to the exhaust system. It can be ground out with a die grinder and carbide burr, but a hole saw works better. It is a very simple job. Should take no more than 15 minutes to do the job once you have the exhaust system apart to access the restriction.
Take a hole saw the same size as the inside diameter of the pipe and run it inside the pipe. You may remove a small portion of the thread for the O2 port but it is not enough to worry about. The exhaust is a low grade stainless steel so use plenty of cutting oil and slow speed.
Thanks to Padgett and Barney Eaton for use of these photos.