The close coupling in the stellarator/torsatron/heliotron between coil design (winding law, peak field, current density, forces), magnetics topology (transform, shear, well depth) and plasma performance (equilibrium, stability, transport, beta) complicates reactor assessment more than for most magnetic confinement systems. In order to provide an additional degree of resolution of this problem for the Modular Stellarator Reactor (MSR), a parametric systems model has been developed and applied. This model reduces key issues associated with plasma performance, first-wall/blanket/shield and coil design to a simple relationship between beta, system geometry and a number of key indicators of overall plant performance. The results of this analysis can then be used to guide more detailed, multidimensional plasma, magnetics and coil design efforts towards technically and economically viable operating regimes. In general, it is shown that average beta values of ≤0.08 may be needed if the MSR approach is to be substantially competitive with other projected approaches to DT magnetic fusion in terms of system power density, mass utilization and cost for total thermal output power around 4.0 GW(th); lower powers will require even higher beta values.