Thanks for the reply Knightshade. I don't really keep up on the 2nd gen stuff so was at a loss about this.
So the 2nd gen IS is probably wideband. The g/f's Scion is wideband. Hell the 01 Tacoma is wideband. Wonder why the 1st gen IS was behind the power curve on that... Maybe using old supra ECU stuff, I donno.
The use of air/fuel sensor and o2 sensor is dependant on how the engineer that developed the power plant wanted it to be monitored. The 2jz series of enginese used four o2 sensors (kind of old school technology). Newer designs like the 2az engine in the scion tC use and air/fuel sensor(wideband) before the catalyst and a regular heated o2 after the catalyst.
The 2IS does have wideband primary O2 sensors, but the secondaries are narrowband. I'm sure the use of widebands for the primaries has to do with improving fuel economy and reducing emissions. While logging AFR via OBDII, I am amazed at how well it maintains stoich even at heavy throttle up to 3K RPMs.