It is..The downside is you don't get that (higher)voltage until the dynamo is spinning faster so the 'cut in' speed for the regulator is higher....
So, if you like chugging round at 35mph on an M20 it doesn't really work...
I ran an M21 with that set up for about 8 years as a daily hack and the dynamo remained reliable....The thing to remember though is that it doesn't mean you can run high wattage halogen bulbs..
I ran a 12volt 35 watt halogen in the headlamp that gave a good bright light...
Running 12 volts on some twins can be problematical...
The magdyno is geared to run the dynamo at a higher speed than some twins...A10s are reasonable, though a kit is available to speed up the dynamo...Norton twins struggle as they run the dynamo even more slowly as standard....
Dynamos armatures that have been rewound to produce 12v (more windings but smaller diameter wire) were not recommended by a friend of mine that was into the theory side of these systems, though I have never tried one myself so can't quote from experience....
I was once told the reason alternators came into general use was because car dynamos couldn't cope with the constant high speed running when motorways were first opened in the UK...I don't know how true that is but it's plausible...Ian
email (option): ian@wright52.plus.com