i'm no expert but o2 spacers shouldn't cause any engine performance whatsover, in fact the main reason you run them is to trick the ecu that everythings fine in case you are having cat converter issues, running headers among other reasons.
Did you erase the codes after installing the spacer? I'd say make sure your o2 sensors in general are ok and disconnect the battery for a few minutes.
Could be a relearning process for the ecu but I highly doubt it.
Looks a lot like a clogged cat issue tho.
__________________
LC Prado 150
4th Gen 4Runner
1GR-FE
OME Springs w Bilstein 5100F&R, Durobumps bumb stops, Toytec 1" BL, Exhaust cam gears with 3° retard, Prado120 Snorkel, AFE Stage 2si, Magnaflow Catback w/ dual side exit 285/70R17 Dynapros MT Energy Suspension Bushings all around
