Those piggy back type systems are generally just signal benders, they take in intpits from say maf sensor and crank trigger and alter it slightly to fool ecu into increasing timingg or changing fueling. They were generally for cars where you were unable to flash new settings into ecu.
Woth fladh tuning you can change hundreds of parametrs directly and you not trying to fool the ecu, you changing actual setting
These cars have flashable ecu you much better doing that than using any type of piggy back system.
http://www.ft86club.com/forums/showthread.php?t=106068