The rapid growth in complexity and diversity of IC designs, design flows and methodologies has resulted in a benchmark-centric culture for evaluation of performance and scalability in physicaldesign algorithm research. Landmark papers in the literature present vertical benchmarks that can be used across multiple design flow stages; artificial benchmarks with characteristics that mimic those of real designs; artificial benchmarks with known optimal solutions; as well as benchmark suites created by major companies from internal designs and/or open-source RTL. However, to our knowledge, there has been no work on horizontal benchmark creation, i.e., the creation of benchmarks that enable maximal, comprehensive assessments across commercial and academic tools at one or more specific design stages. Typically, the creation of horizontal benchmarks is limited by mismatches in data models, netlist formats, technology files, library granularity, etc. across different tools, technologies, and benchmark suites. In this paper, we describe methodology and robust infrastructure for "horizontal benchmark extension" that permits maximal leverage of benchmark suites and technologies in "apples-to-apples" assessment of both industry and academic optimizers. We demonstrate horizontal benchmark extensions, and the assessments that are thus enabled, in two well-studied domains: place-and-route (four combinations of academic placers/routers, and two commercial P&R tools) and gate sizing (two academic sizers, and three commercial tools). We also point out several issues and precepts for horizontal benchmark enablement.