My normal grocery load is about 45 lbs including groceries and trailer weight. Compared to the unloaded bike over the same route, the load costs me about 1 or 2 gears on hills. I pull about the same gear on the flats. And downhill...look out!
With the Xtracycle and similar load, it's about the same. I have to drop down 1 or 2 gears with the load. That's fine unless it's a hill that I already have to take in the lowest gear unloaded...
Eric