When I had my scope I found a lot of noise coming in right around the ignition event. I never had this tach spike issue with the 2.2 board, maybe there is some crosstalk somewhere between traces.
The best workaround for that is to shield the crap out of everything. I'm talking coax everywhere. Adding software fixes couldn't hurt, either. Although I don't know much about the current state of the decoder, I'm sure there is room for improvement on a better definition on what is a tach spike and what is a rising or falling edge coming in from the sensor. I don't want to learn a new language (assembly) to fix it. A port of the -extra code to the MS2 platform (C language which I can code in) is LONG OVERDUE. The handful of developers are just too goddamn lazy to do it, mainly because they are not being paid and do it and all of this gets done in their spare time. I would be dragging my feet on it too if it was my "problem".
