In theory, ISO is a standard, based on the exposure required to give a negative density of a certain amount over base plus fog when developed in a specified developer at a specified temperature for a specified time using a specified agitation. Vary any of those parameters, and expect a slightly different result. Then factor in that shutter speeds are not guaranteed to be totally accurate, and even aperture markings can be incorrect (Ansel Adams calibrated his apertures for this reason!). In other words, you can't reasonably expect to get the same results in term of density as the lab did.
The difference between 160 and 200 ISO in real world terms is the difference between a shutter speed of 1/100 and 1/80. I suspect that for many people, the two would be close enough.
I've not used Fomapan 200, so no voice of experience. My own practice, slapdash as always, is that with larger negatives, the penalties of overexposure are less than 35mm (less sharpness, larger grain) and the penalties of underexposure just as severe - loss of shadow detail, low contrast in the shadow areas - and I always downrate film to add in a safety factor. I use 80 rather than 125 for FP4 for example.
In doing this, I'm following an historical precedent. At the start of the 1960s, film speeds had a safety factor built in. In the early 1960s, around 1962, the safety factor was dropped and film speeds doubled (literally) overnight, with no changes to the film.
