Geoff, aluminum O/A doesn't make much sense to me for anything but bodywork, unless that's the only welding kit you've got falling to hand. There are good reasons why they invented TIG and it has so many positive features (though price is often not one of them). I've got a Miller 250DX and it is usually my first choice if I need to weld something. But O/A does have a niche with the panel beaters.
Kent was a restorer at the Harrah Auto Museum and I think one of the cars he did there took overall honors at the Pebble Beach concours one year. He knows his stuff, and he has put a lot of effort and thought into the tools he's developed.
One of the guys in the class I took from Kent had a hot rod shop in the midwest. He was having a horrible time with O/A alloy welding until he mentioned that he'd left his reading glasses at the hotel. I loaned him my goggles with the magnifiers in them and he immediately began welding .062" sheet as though he'd been doing it for years. It was pretty disheartening for those of use who were struggling.
I once got to try using a (large) torch running oxy/hydrogen and that seemed to be a big help. The O/H flame is cooler and it was easier to avoid overheating the metal.
As you say the broader HAZ on O/A keeps things workable which is important when there's going to be more pounding/rolling/planishing/bending being done after welding. When done by the right hands it also can be a very speedy weld vs TIG.
Every pushbike I've ridden has always been grossly underpowered.
cheers,
Michael