Here are my observations going from the Porterfield R4-S street pads to the Porterfield R-4 track pads on my 2007 Aston Martin V8 Vantage. Most were in line with what I expected, but the extent of certain differences was quite surprising.
Brake pads vary greatly in a few key ways. Initial bite, braking performance when cold or hot, rotor "friendliness" (how much they chew up brake rotors), dust, and noise are the main concerns for most of us that pay attention to such things. I've been running Porterfield's R4-S brake pads on my V8 Vantage for several months, including track days, and today I'm swapping them out for the more track-oriented R4 pad.