I think there is some major confusion here. Project Mu is a brand that makes many different compounds. They make street pads designed for street use only (NS / NS400), track pads designed for track use only (Racing 999) and just about everything in between.
It is important to understand how to choose a pad that suits your situation best. Diver experience, vehicle weight, vehicle power, tires used, and the track driven on are all factors that must be considered when choosing the correct pads for track use. Road America is especially brutal on brakes compared to many other tracks and the ISF is a heavy and powerful car that is hard on brakes. Combine the two with an inappropriate pad selection and you will heat the pad way beyond it's MOT (maximum operating temperature) resulting in pad fade, extremely fast pad wear and ultimately a scary and frustrating situation.
Here is a chart from Project Mu that shows how some of their different compounds compare:
I believe the Club Racers are a derivative of the Racing 777 and H16-03 from their racing compounds. They are not listed on that chart but they are indeed a track specific pad that can be driven on the street (I daily drive mine in between track events) if you can handle a little noise from time to time.
Since The NS400, HC+800 and Club Racer are the only relevant compounds for the ISF community we can focus on those. As I mentioned in the other thread The Club Racer is the only pad specifically designed for track use and will last for several track days even on a heavy high power car like the ISF. It is in a completely different league compared to the HC+800 and in a completely different world compared to the NS400. The HC+800 is an aggressive street pad that can take some track abuse from newer drivers on street tires (or lighter cars) but can be overheated if you have an experienced driver really pushing them on a heavier car. NS400 is a high end street pad that should not be used on a track in an ISF.
Project Mu NS400 compounds are developed specifically for street use. The NS400 pads are reliable and guaranteed to outperform OEM pads for a reasonable cost. NS400 pads provide outstanding initial stopping power which is of prime importance on the street. Made from asbestos-free brake compounds, NS400 street pads have excellent low dust and squeal properties. Use this pad to get the most out of your street car.
Project Mu HC+800 Brake Pads are a premium performance street, race, and rally brake pad. HC800 Brake Pads have more bite than both NS400 and B-Spec and has excellent fade resistance. Capable of operating at temperatures up to 800° (1,472°f), this pad will handle almost all serious braking conditions. Along with temperature capabilities, HC+800 is characterized as a “drivers pad” with unmatched driver feel and control. HC800 is a genuine ‘crossover’ pad suitable for both road and race. It is extremely capable in competition environments, so use this pad if you are involved in serious track day, rally, etc.
Club Racer description:
Club Racer is the new pad from Project-Mu for club racing and rally enthusiasts with seriously fast cars. It is a brake pad that offers the very best in performance, feel and release. This pad really punches above it's weight and will push aside many other over-priced race pads in many race and rally applications. It is based on the DNA of Project-Mu's ultra successful H16-03 motorsport pad which is one of the world's best race pads today. Club Racer is the pad to use of your car spends a majority of it's time on the track or in a rally at anything up to full professional motorsport.
I should also mention that although Project Mu claims the HC+800 is capable of track use, they work alright but they will wear much faster when they get that hot. It is a great pad for newer track drivers only doing a few events per year but drivers who go to the track more often will want something more appropriate (Club Racer, XP12, PFC 11, etc.)