Note that the hdf5 instruments added to mantid were created via the
python-nexus-utilities tools at tip
ba1ed8b387258f6841f1af71ee0ea9e098babedb

Also note that i had to add code to respond to runtime knowldge about
exact integer representations in the file.

From the current timings for equivalent instrument creation times
        InstrumentDefinitionParser    NexusGeometryParser
WISH    2200 ms                       500 ms
SANS2D  610 ms                        80 ms
LOKI    200 ms                        10 ms

The majority of these are available via the respective Performance Tests
for the tests suites.