  The recently developed JWST Exoplanet Observation Simulator (JexoSim)
simulates transit spectroscopic observations of exoplanets by JWST with each of
its four instruments using a time-domain approach. Previously we reported the
validation of JexoSim against Pandexo and instrument team simulators. In the
present study, we report a substantially enhanced version, JexoSim 2.0, which
improves on the original version through incorporation of new noise sources,
enhanced treatment of stellar and planetary signals and instrumental effects,
as well as improved user-operability and optimisations for increased speed and
efficiency. A near complete set of instrument modes for exoplanet time-series
observations is now included. In this paper we report the implementation of
JexoSim 2.0 and assess performance metrics for JWST in end-member scenarios
using the hot Jupiter HD 209458 b and the mini-Neptune K2-18 b. We show how
JexoSim can be used to compare performance across the different JWST
instruments, selecting an optimal combination of instrument and subarray modes,
producing synthetic transmission spectra for each planet. These studies
indicate that the 1.4 {\mu}m water feature detected in the atmosphere of K2-18
b using the Hubble WFC3 might be observable in just one transit observation
with JWST with either NIRISS or NIRSpec. JexoSim 2.0 can be used to investigate
the impact of complex noise and systematic effects on the final spectrum, plan
observations and test the feasibility of novel science cases for JWST. It can
also be customised for other astrophysical applications beyond exoplanet
spectroscopy. JexoSim 2.0 is now available for use by the scientific community.
