Things to consider. Copied from PHP's site
Power Hungry Performance - Frequently Asked Questions
Do you offer a "decel" or "exhaust brake" calibration?
Simply put, no, we don't because it goes against our business philosophy which, in part, is to provide safe, reliable products to our customers.
For those who don't know, this is a calibration that uses the EBV (Exhaust Backpressure Valve) as a makeshift exhaust brake. While this setup may work reasonably well for lightly loaded vehicles, we generally do not recommend this type of calibration... at least not for automatic transmission vehicles. There are a few reasons we don't recommend this.
First, in order to transfer the energy of the moving vehicle to the engine during deceleration, two things must occur: the torque converter must remain locked, and the coast clutch must be engaged. The problem with this is that the coast clutch on a stock transmission only has 3 friction plates, and these are smaller than the standard forward clutches. On 2001 and later transmissions, you also run the possibility of damaging the intermediate sprag or "diode," which will result in an immediate transmission teardown. Again, lightly loaded vehicles would have less of a problem with this, but the general idea of an exhaust brake is to be able to slow larger loads without overheating the standard brake system. It is these types of loads that can cause transmission damage. Obviously, a manual transmission would be free from these types of situations.
Second, whenever you increase exhaust backpressure, you need to make sure that the exhaust valve springs are capable of preventing the pressure in the exhaust system from lifting the exhaust valve from the valve seat, as this would result in a collision with the piston. Normal valve spring seat pressure is 70-75 PSI for NEW valve springs and deteriorates from there. Given that the surface area of the back of the exhaust valve is approximately 1.9 square inches, it would take only 40 PSI of exhaust pressure to lift the valve off the seat, even with new valve springs.
To put it simply, if you plan to use an exhaust brake, either through the use of the EBV or by purchasing an aftermarket stand alone unit, you will need to consider the condition of your transmission and exhaust valve springs in order to ensure safe, reliable operation.