I am unsure how big a difference you would get on an HD Panny. I have not seen one in the flesh. Once I am allowed to I will discuss the DVI out of the Lumagens when partnered with HDMI inputs etc against the same inputs and analogue outs to the plasma.
I have discussed the differences before when I used the Key Digital scaler to do this. With that device I ended up using analogue and continue to. This could all change of course.
The only points i would add are that as you are obviously comfortable with the extra features/configurability/complexity of an HCPC solution then that is a good route to go down vs seperate dedicated units. But, as always its horses for courses , we supply both and each has its own appeal...
Finally, I would agree that using VGA vs DVI (particularly on a relatively small (compared to a PJ) display) is a close run thing. It makes much less difference to the overal picture quality than say using analogue vs digital from a DVD source.