I believe Richards caution is wise. Eg, if you try to draw 200A from a 150A alternator, I think the alternator will try to supply 200A even though it won't be able to, and you risk burning the alternator up. If you are only at idle, the alt may not be turning fast enough to burn itself out, so you may be okay. Having said that, I see people using their winch with the engine running reasonably often, so the danger might not be that high.
But you won't be drawing 200A direct from the alternator if you've got even the main battery connected (which you will as it's obviously wired into the vehicle). Some of the current draw will come from the battery.
If you were purely loading up the alternator then surely any vehicle with a winch would require a dedicated battery to run it so won't burn the alternator out?
The factory winch for a P38 was a Warn XD9000 or something along those lines and the current rating on that is over 400A at max line pull. The specs for it recommend a minimum 650CCA battery for winching, but again in the LR fitting documents, there's no talk of requiring a second battery to run it - even on the earlier GEMS vehicles where they had a 120A alternator.
As I mentioned, I put a second battery/split charge in mine in the thought that at some point I'd fit the winch, once I found one I liked and would fit, but not because I was worried about the current capacity, but more to give the system a helping hand and I wanted to run other things off of it too.