[Christ]

It's probably not the cheapest solution, but it has water inlets and outlets already, and is designed to run water through the ports to cool intake air for turbo/supercharging applications, however, if you were to pipe your exhaust through it, you could replace one or more of the mufflers/resonators on your car, as well as extract a good bit of heat from the exhaust into the coolant mix via the series of small passages in the body of the heat exchanger, which is specifically designed to remove heat from the passing air and deliver it into water.

It's practically designed for this application, just in a slightly different sense.

That particular unit, from the linked website (pic is link) runs $169. You can probably find/make something cheaper, especially if you can weld aluminum and find a junkyard with turbo parts. DSM cars typically came with small side-mount intercoolers that might work effectively if the cooling air portion were blocked and bungs installed to flow liquid cooling medium through the heat exchanger.

I also have no idea what type of restriction there would be in such a setup... however, I expect minimal, again because of the designed intent of the item. Pressure drop across an intercooler is usually very little, even after cooling.

That is a topic to which we should be paying attention, though. Engines are designed to run with a specific exhaust velocity and volume, and the volume that the exhaust occupies will be less once its' cooled, as well as the velocity having slowed. This could have an adverse effect on engine/emissions controls and performance. Something to think about/keep an eye out for.