The article is not a bad one, it is one of those I looked at when first evaluating whether or not tankless made financial sense for me. If you want fast payback, use a 1.5-1.6 gpm showerhead instead. Cost is around $30-$50 depending on what you choose. Rate of return is about 100% annually.
However, good luck "packing a home with insulation" and getting a 30% annual return unless it has very little insulation to begin with. While I've done a number of insulation projects, the areas that need it the most often can't be fixed...or if they can, not so easily. Good luck adding insulation thickness cheaply in a cathedral ceiling or uninsulated basement space. And from my own projects insulation is just a minor part of the cost where energy savings are highest. By the time you remove the drywall, insulate, put up new drywall and paint the cost can be quite high and the time investment considerable. The areas that are easiest to add insulation to are open attics...which already tend to have passably high R values to begin with.
It's too bad that the average builder does such a poor job of insulating a home, because it is a lot more expensive to retrofit in places they missed (or skipped and concealed.) Ditto for leaky, uninsulated HVAC duct work.
Windows? Now there is a poor return on investment...unless they have to be replaced anyway. It is really expensive compared to selecting a higher efficiency water heater or higher efficiency furnace. Heck, the payout on a new fridge is probably quicker...and that is really slow. But when replacing anyway, higher efficiency windows make sense.
Besides showerheads, the really easy money is in things like CFL's.