  We simulate the performance of a new type of instrument, a Superconducting
Multi-Object Spectrograph (SuperMOS), that uses Microwave Kinetic Inductance
Detectors (MKIDs). MKIDs, a new detector technology, feature good QE in the
UVOIR, can count individual photons with microsecond timing accuracy and, like
X-ray calorimeters, determine their energy to several percent. The performance
of Giga-z, a SuperMOS designed for wide field imaging follow-up observations,
is evaluated using simulated observations of the COSMOS mock catalog with an
array of 100,000 R_{423 nm} = E/\Delta E = 30 MKID pixels. We compare our
results against a simultaneous simulation of LSST observations. In three years
on a dedicated 4 m-class telescope, Giga-z could observe ~ 2 billion galaxies,
yielding a low resolution spectral energy distribution (SED) spanning 350 -
1350 nm for each; 1000 times the number measured with any currently proposed
LSST spectroscopic follow-up, at a fraction of the cost and time. Giga-z would
provide redshifts for galaxies up to z ~ 6 with magnitudes m_i < 25, with
accuracy \sigma_{\Delta z/(1+z)} = 0.03 for the whole sample, and
\sigma_{\Delta z/(1+z)} = 0.007 for a select subset. We also find catastrophic
failure rates and biases that are consistently lower than for LSST. The added
constraint on Dark Energy parameters for WL+CMB by Giga-z using the FoMSWG
default model is equivalent to multiplying the LSST Fisher matrix by a factor
of \alpha = 1.27 (w_p), 1.53 (w_a), or 1.98 (\Delta \gamma). This is equivalent
to multiplying both the LSST coverage area and the training sets by \alpha, and
reducing all systematics by a factor of 1/sqrt(\alpha), advantages that are
robust to even more extreme models of intrinsic alignment.
