I am kind of a DIY'er myself but I don't think I would even want a unit with a heavier flywheel. A lot of the training DVD's involve short interval work (accelleration sets like 10 sec on/10 sec off, 30-60 second intervals, etc.) which would be frustrating with a heavier flywheel. It would probably make the unit feel a little more realistic though for longer steady state riding with a longer coast-down time.
I bet KK doesn't sell too many of the units with the heavy flywheel. That definitely wouldn't be an upgrade for me.