Personally I would take a compact, well designed gun over a more powerful oversized gun any day of the week.
If the Webley's or the Magnum were fatter or 2" longer they would become monstrosities in my eyes.
400 or so fps in .177 is plenty for the purposes of almost all air pistols - more power seldom makes a better gun.

Look to the Webley Senior for inspiration - remarkably compact (much smaller than a Tempest) and beautifully made and plenty of poke for the size typically running at about 410-420fps - in fact with a better trigger and sights it would be bloody perfect IMO.