It's been done, and there is a lot of info on these out there.
R160, R180 and R200 are not identical housings, in fact the R200 is a fair bit bigger afaik. What you are probably thinking of is that an R160 in a subaru is the same as an R160 in a datto, which is correct except for things like the half-shafts. I think LSD is more common (but still not that common) in subarus.
You will find R200's in dattos as well as subies, too.
It depends what you're after, if you are looking for a change in ratio then you could just swap another R160 in, if you need something stronger then you might want an R180 or R200 but you might still want one from a datto for the half-shafts and possibly the axle assembly, as I am not sure you'll be able to use your exisiting R160 half shafts in an R180 or R200.
If on the other hand you are after LSD, good luck; I think they are pretty sought-after.
Anyway there is heaps of info on all this, eg:
http://www.google.com.au/search?q=R160+ ... =firefox-a
This is not legal advice.