This paper is concerned with the application of formal optimisation methods to the design of mixed-granularity FPGAs. In particular, we investigate the appropriate mix and floorplan of heterogeneous elements: multipliers, RAMs, and LUT-based logic, in order to maximise the performance of a set of DSP benchmark applications, given a fixed silicon budget. We extend our previous mathematical programming framework by proposing a novel set of heuristics, capable of providing upper bounds on the achievable reconfigurable-tofixed- logic performance ratio. Our results provide, for the first time, quantifications of the optimal performance/areaenhancing capability of multipliers and RAM blocks within a system context, and indicate that only a minimal performance benefit can be achieved over Virtex II by re-organising the device floorplan, when using optimal technology mapping.