I don't know FePO4 in SubC Sice (too bad). I use a 2S1P Pack with 2300mAh and 6V. They are much bigger than SubC cells, but 2 cells will fit in almost any 12th chassis. I think I posted a picture of my car in this thread.
I never ran with transponder since I use this cells, but I think its a little bit slower than normal 4cell modified.
For me as a hobbyracer, who just participates at some clubraces, is it an inexpensive alternative to Nimhs with many advantages.