Actually, the reason that the Magnette was released with the B Series
engine and the Wolseley with the XPAW engine was nothing to do with
giving the Magnette more power, although this was a desirable side
effect of that decision.  Both cars were designed prior to the
Nuffield/Austin merger, but the Wolseley was ready ahead of the
Magnette and it was decided to release it with the Nuffield designed
engine and change over to the B Series at a later date (which resulted
in the introduction of the 15/50 a few years later - basically a
FourFortyFour with a B Series and a handful of minor cosmetic
changes).  The Magnette was still under development at the time of the
merger and it was relatively straightforward to change the design to
accomodate the B Series engine.

If you're looking for more information about these cars, the following
web page could be worth checking out:

http://www.mgcars.org.uk/mgccz/Articles/articles.htm