You can't really starve a diesel engine - although air is metered, it's not throttled. There is no throttle, in fact.
The biggest single thing that affects fuel economy is driving style. Remapping is a good idea - even an optimised performance map will help economy provided you don't go and use all the extra torque on offer, all of the time.
An eco map is still a performance gain map but the top end power curve is smoothed off, which encourages earlier upchanges. You can achieve the same effect with a performance map and adjust your right foot.
Some Toyota common rail diesels can be hard to remap, however. You might have to resort to a tuning box but be very careful here - even the very best will not work well with cars that are equipped with particle filters.
You cannot alter the pump on common rail engines as you could with earlier diesel engines. The pump is just that, a pump - it plays no part in fuel injection timing or injector dwell time.
19mpg does sound very low to me though, these should manage about 30mpg in mixed usage. - Is everything else working correctly?
