I believe that Jean L or someone in his family has replaced an axle using the Flexride. I'll give him a heads up to your thread.
As for the center of gravity. If you keep the center of the wheel in the same position from front to rear the center of gravity will be the same. The wheel position is what determines the center of gravity. The flexride has a shorter arm which will move the wheel back a bit if installed in the leading arm configuration.
I explain how I switched from a leading arm to a trailing arm in this lengthy post
Yes I did consider using a flexride in trailing arm. Final decision was based on what worked best for heights, positioning, tires
etc. to meet my needs. You should note that both Flexride and Dexter have differing recomended start angles. Those affect your height (more) and position of the spindle (less).
I question wanting to replace a single torsion unit on the flexride. IMO unless there are extenuating circumstances if it is time to replace one, you most likely need to replace both or the entire axle. Whatever you do, consider the benifits of using the bolt on brackets for either axle.