While I was at it, I installed a Dakota Digital SGI-8 E tach converter, which corrected my tachometer reading 33% high with no apparent latency. Following the wiring diagram above, the tach signal I needed to interrupt is E26 from the RSX ECU and is also conveniently the same color as the tach signal on the Insight harness.
Still need to sort out the temperature gauge. I believe the sensor is just a varistor, but the RSX sensor uses a different resistance so the Insight gauge jumps from "low" to "overheating" a few minutes into my drive.