First of all, welcome to the forums. In regards to your questions, I think the biggest question is why? What would you like to modify on the PM6 LED? I'm a big fan of the PM6 LED as Pelican has done a great job in selecting LEDs for production (they're all pretty white compared to what you normally get with a production Luxeon light), the 1W throws extremely well in the Pelican reflector (which is one of the best reflectors out there), and it gives you (according to the Pelican website) 40 hours of burn time. Did you want to change the circuitry? I think there was some criticism about that.
ps...I'm not criticizing your questions, I just think that the questions are easier to answer if you let the modders know what you wanted to have done.
edited to add:
1. You can change it to a 5watt module to increase output, but the 5watters are harder to find in a good bin code and don't throw as well as the 1w.
2. Yes--lots of modders out there (unfortunately I'm not one of them) that could help you out.
3. I don't think there's any real undesirable effects. The 5watt may run hotter, but my 5watt doesn't run hot enough to bother me that much--at least not in comparison to the 1w.
4. I think this may be cost prohibitive for what you'd end up with in comparison to what the PM6 already is--but that doesn't usually stop any of us /ubbthreads/images/graemlins/grin.gif .
Sorry for the ramble--just wanted to answer the questions more completely. And everything above is completely my opinion and I'm sure there will be people who feel otherwise. I hope that helps you somewhat.