wgpu supports both a SPIR-V passthrough mode [0] and a regular SPIR-V mode where it will get some transformations added to it (e.g. bounds checks are added on backends without VK_KHR_robustness). I don't believe Dawn has a similar passthrough mode, only a SPIR-V -> transform -> SPIR-V path.
[0] https://docs.rs/wgpu/latest/wgpu/struct.Features.html#associ...