I rode a freinds xtra and notices the wheelbase thing a little bit, then i noticed he had like 40lbs in the back. And i thought it was just 'a little handleing' difference.
Speed only matters when there's an uphill with the extra weight, course the conversions only 10lbs. Theres no extra wheel so no extra rolling resistance and the drivetrain is the same so no extra mechanical resistance. If you put an extra on a good bike you should have a good cruising speed, a little wind resistance aside.
There was a blog by a guy riding cross country on a road bike with an extra, forget the name but on crazyguyonabike.