I have an HMF exhaust on my 2010 RS. When I first installed it, I did not have a fuel modifier or K&N filter. It ran great just like that, however, I noticed a lot of popping on deceleration. I for one, like the popping but it was occuring more than I liked. I purchased the modifier from HMF, a K&N air filter, and installed both in one afternoon. The popping on decel was immediately resolved with just a few pops every now and then which was perfect for me. The air filter was installed in the stock airbox (which is also a great time to clean all the oil out of the box) and installed an oil catch to prevent engine oil being blown into the airbox but allowing pressure to vent.
The fuel modifier is pre-programmed with fuel mappings from HMF for the RS. You can change the settings as you please to increase or decrease the amount of fuel going into the engine. Someone correct me if I am wrong on this next part: As I understand it, the K&N allows much more air into the engine, therefore allowing it to run leaner, increases engine temperature and makes popping on decel. Adding more fuel enriches the mixture and lowers the temperature of the engine. I would say if you only install the pipe and leave everything else stock, you should be just fine. If you add the K&N, then I would recommend you add the modifier as well.
You can read my other posts about my experience with the fuel modifier from another thread. Check my profile to find it.