The unpredictable worst-case timing behavior of multicore architectures has been the biggest stumbling block for a widespread use of multicores in hard real-time systems. A great deal of research effort has been devoted to address the issue. Among others, the development of a new multicore architecture has emerged as an attractive solution because it can eliminate the unpredictable interference sources in the first place. This opens a new possibility of system-level optimizations with multicore-based hard real-time systems. To address this issue, we propose a new perspective of WCET model called tunable WCET, in which the WCETs of tasks are elastically adjusted according to the optimal shared resource allocation and arbitration methods. For this, we propose novel WCET-aware harmonic round-robin bus scheduling and two-level cache partitioning method. We present a mixed integer linear programming formulation as the solution to the optimization of tunable WCETs. Our experimental results show that the proposed methods can significantly lower overall system utilizations.