This is the second thread on this topic in as many days. So here's the same answer again. 2M, you know more about electronics than I'll ever learn, but this is based on experience.
Spiking due to electronic noise is a common problem with aftermarket tachs. Expensive ones like the Veypor data logger from Nonlinear Engineering have programmable filters. Cheap ones like Trail Tech and Acewell instruments don't, but the solution is simple. Install a 1 megohm resistor in the line that carries the signal from coil to instrument right after the coil. If you still get spikes, add resistors. If response is to slow, try less resistance. That's all you need to do.
I should add that this is only an issue with breaker point and the cheaper aftermarket electronic ignitions that are just about all that's still available for the XS650. The Probe ignition that I currently use has a tach signal output from the igniter box, and no damping is needed. Because XS650 and other vintage Jap bike owners were unwilling to pay an extra $50 or so for quality, Probe Engineering has discontinued production of motorcycle ignitions in favor of operations that carry their own weight. If you have a Boyer ignition, remember that when your timing plate warps.