Fine tuning of air/ fuel ratios and ignition advance has never and can never be a substitute for physical modifications that improve the engines breathing, all they can do is optimism the use if the air that can be drawn in.
As can be seen in the Fuel/ Air ratio chart linked below, the sprockets allow the boxer to take in more air up to about 7500rpm. More air in equals more power and torque out, that should be simple enough to understand.
http://www.bikeboy.org/r1100sboxercamsproc.html
Remapping fuel and ignition has no effect on engine breathing so can NEVER provide what the sprockets do. An optimized tune with the sprockets installed will ALWAYS provide more power and torque below about 7000rpm than an optimized tune without the sprockets.
Those who want the best performance from their boxer below about 7000rpm have no choice but to install Lennies sprockets and invoke a richer fuel map, either via an ECU remap, add on fuel nanny or via the much simpler and cheaper air temperature sensor trick. A remap alone can never provide the same result.
As for products like Rexxer being the way of the future, I dont see it. I expect future motorcycle ECU's to be so locked down by the manufacturer that remapping will be effectively impossible.