Here goes man... As you know I have done the 5efe conversion in my EP91 same as you. The engine wont run mate if u plug in the 5efe ecu, it starts then dies immediately after. This is due to the factory immobiliser in the 4efe ecu working with the 4efe loom. It doesnt seem to be compatible with the 5efe ecu due to this. I went to a diagnostics place and they were known to reprogram toyota ecu's, so I went there and he checked it out. Seems that the 5efe ecu can only be reset/reprogrammed by Toyota = very expensive. I dont know if it is as simple as deactivating or bypassing the factory immob but that could be an option if you are dying to try a 5efe ecu. Im also having thoughts about it, as I want a proper car alarm also and it will have its own immobiliser, hence no need for the factory one. The 5efe actually runs pretty well with the 4efe ecu which is not surprising as they are both very similar engines with minor differences. However as you know, the 5efe is a refreshing update to the ep91 and really makes a nice difference don't it, you don't have to rev as hard or drive as fast as the experience is naturally more torquier and the starlet moves much more strongly on the road and stably. One concern I have is that the 5efe has a lower compression ratio of 9.4:1 compared the 4efe's 9:8:1. I believe the actual fuel maps and timing are very similar, but there is no proof without proper evidence. For me this is not a problem as I will be running a different engine management setup later on so I can fine tune whatever I want. You'll have to ask someone else about the key chip thing, you may need to change the ignition barrel too possibly. Hope this helps. Joz