Cameron - there's a noticeable drop in temp with the coating. My previous Cobra also had HPC, and you could touch the headers only minutes after turning the engine off. The current car runs the pipes REALLY close to the footbox, as I lengthened the footbox by 20cm. As a result, I'm now using a header wrap as well. I know that this is supposed to speed up corrosion of the pipes, but if I have to replace the pipes after 4 or 5 years, it's worth it for the lowered footwell temps. In fact, with the extra heatshielding that I attached to the footbox walls, I literally cannot feel any temperature at all inside the car.
Peter, go with the sidepipes.
Other than the constant worry about passengers or curious onlookers burning themselves on the pipes, the sound of sidepipes is fantastic !!! You'll have a whole lot more ground clearance by running the pipes out the side, too.