The 800 is of course 2 x D250's in dual mono. Although I believe the old D500 was just about the only thing Devialet ever quoted at 8 ohms, so the power difference D-Premier D500 to latest D800 is not as big as it appears. Is it a BIG step up? Hard to say. One man's massive improvement is another man's, well ok it is maybe slightly better. I can only speak for myself, my own ears and my own system, but the improvement was enough for me to buy the companion new.
As another thought..... There is potentially a good improvement to be had from SAM. This is obviously not an option with the D-Premier. Will the G1's ever get SAM? I have no idea, but it might be worth checking this with Devialet. If they will eventually get it, I would think hard about going for a D250 (with a view to go D800) or maybe even a D400. If you are reasonably sure that the G1's won't get SAM, then two D-Premiers would be a fine option I think.
As another thought..... There is potentially a good improvement to be had from SAM. This is obviously not an option with the D-Premier. Will the G1's ever get SAM? I have no idea, but it might be worth checking this with Devialet. If they will eventually get it, I would think hard about going for a D250 (with a view to go D800) or maybe even a D400. If you are reasonably sure that the G1's won't get SAM, then two D-Premiers would be a fine option I think.