Peugeot bought out Citroen in 1974 which is very much the reason why Peugeots and Citroens share engines in many of their models, all made by the same manufacturer (PSA). The Citroen AX appeared in 1987 and had the engines which were introduced to the 205 range in 1988. The Citroen AX was later 'cloned' by Peugeot as the 106 (AX's sister model) then the 106 was again copied as the Saxo.
Anyway, the performance car of the AX range was the GT (later GTi) which had an impressive 0-62mph time of 9.5 seconds (may be faster in reality). The engine powering this model was the same as that in the XS. The improved acceleration was due to the lighter bodyshell of the AX.
An interesting point about the 205 XS is that it is able to stay close behind a 1.6 GTi for some distance due to the aluminium block engine of the XS being lighter than the 1.6 engine. The lower gear ratios in the XS also help in improving acceleration time (5th gear can be comfortably used at speeds as low as 35mph).
Not to mention that both cars had the twin-choke carburettor.(For those of you who don't know what this means, upon pressing the pedal down to a certain point, a second jet of petrol gets set off, increasing the engine power upon demand. It does give quite a kick when accelerating hard and is nice to keep trying it out, though be prepared for more regular visits to the petrol pumps!)

Below I am hoping to devise a chart which I hope will be of use to many owners of sporty Peugeot and Citroens when finding parts for their cars. It will detail all Peugeot/Citroen engines over 1294cc (excluding automatics & normally aspirated diesels), their capacity, bhp, engine code and Peugeot and Citroen cars they are found in. (I did try to do all engines, but it became a very difficult task, so I am specialising more!)
I am not testifying to this information being 100% accurate, so please...
if you know of and are certain about any amendments or additions to add to this table, then please e-mail me and I shall include them here. Hopefully this will be a resource which many people can refer to in future as even Peugeot themselves don't have details on which engines relate to which cars. This table will involve a lot of research to compile it. I really need help on this one.
If anyone actually has a list similar or knows of one on the web, again please e-mail me and I can possibly see if it can be made available here too. Cheers.