The transfer case will be fine. It's a constant meshed planetary gear system, I'm sure it can handle the load from a supercharger. The rumored (or whatever) spec limit on the transfer case is very close to the output of the 3.0L 330xi engine which seems like a dumb move on BMW's part, you don't make a part that walks that close to the line for failure hence why I don't think it's true.
I would be more concerned with the transmission, clutch, differential, brakes, suspension, tires, cooling, belts, and all that stuff before the transfer case...
Lastly, don't waste your money on supercharging your 325xi IMO. If you want power, sell it and get an M3. It's designed for that power from the get go and you don't have to mess around with changing/maintaining it.