1600i ones are much lighter for a start because the crossmember is alloy.
The roll bar is thicker than the S1, so should be stiffer (Although the S1 has a solid bar as opposed to the hollow one on the 1600i).
1600i ones have two different types of tension strut/ARB clamp. One has a rubber bush inside, the other is a solid metal to metal type clamp same as the S1. I dont know if there is any difference in performance between the clamps. After checking a few 1600i's out at ford fair, the rubber bush clamps appeared to be fitted to the later cars IIRC.
As far as i know, you cant get a poly equivalent for the clamp rubber bush. But you can get poly bushes for the ARB to crossmember mountings.
There will be no difference to the suspension geometry, although i would get it checked due to the adjustability of the tension struts. (They may have been altered before.) These only alter castor anyway. The camber differences between early 2 bolt shells and later shells is made by relocation of the strut top.
The tension strut to crossmember mounts are also different. Theyre like a rod end with a rubber boot on the S1, but like a ball joint assembly on the 1600i.
There. Thats everything i know. Hope it helps!