Just to add on the subject of why the 1.3MJ has a lower output in the 500 than in other cars, gearbox questions not withstanding, I think must be at least influenced by marketing.
As the diesel and the 1.4 are the same price, no-one would buy the 1.4 if the diesel was faster as well as using 50% less fuel. But without a turbo there probably isn't much more to squeeze from that motor, which would also put the price up quite a bit and in any case venture into Arbarth teritory.
Just my reckoning