1) that the engine would feel lumpy on a cold start (overnight at less than 10C) or hard acceleration at high RPM. It was most noticeable when the engine was running in open-loop mode, i.e. it was ignoring the data from the O2 sensors and relying on the MAF & MAP etc to get the fuel mix right. When the O2 sensors kicked in and it went closed loop (about 1-2mins from a cold start) the ECU got better at guessing the fuel mix based on the output of the engine rather than the input. Which led me to do the test in point 2 below.

2) using a can of MAF sensor cleaner. If you've never used on of these its basically an aerosol degreaser that is designed to dislodge muck and quickly evaporate leaving no residue. You run the engine at about 2000 rpm, fire it into the air intake after the air filter in short bursts, it should quickly coat the MAF sensor with fluid, which will cause it to send random readings to the ECU who in turn will almost stall the engine trying to compensate. Once the cleaner has evaporated (a few seconds later) the MAF should start sending sensible readings and the engine will pick up again to 2000rpm. I found that when I did this on bank 2 the engine behaved as predicted, nearly stalling, however when I did it on bank 1 it made little difference to the output of the MAF and therefore the engine speed. Quick replacement of the MAFs and it was all back to normal, well normalish, because by now the cats were screwed.....