Ok, now this all's got me thinking...does anyone ever "build" these engines?
Ie., bore them (I think GoWesty does this), increase compression, change cam
characteristics, larger valves, tweaked ECU, turbo,and all the other stuff
that goes into a higher performance engine?

If not, why not? Again, if I'm not mistaken, this happens all the time in
the air-cooled world. It seems like the waterboxer low end, crankshaft,
bearings, etc., is pretty sturdy, so that probably wouldn't be an issue. If
people are spending mega bucks to do conversions, couldn't they put the same
into the waterboxer and end up with twice the power (of the wbx, not the
conversion engine)?

Just musing, late at night.