Re: how bad is this?
hmmm...Mr. Goodwrench...oh well..
The extractors work like a hose syphon. Ya know...the little bottle attached to the garden hose with the chemicals in it....or just like a sand blaster... the flow of the exhaust draws the blow by vapor into the exhaust stream. The backfire valves are just that....they prevent exhaust flow from reaching the upstream plumbing during a "shock event" like a backfire.
These guys are assuming the worst case scenereo for your problem. The detonation can just be oil in the intake stream. This is not an alien event. Oil does that. It doesn't fully combust and therefore leaves a medium to support combustion early in the cycle.
This is not to say that you don't have bad rings. This is almost a given. But that does not mean that you have to put out the enormous amount of cash (even @$150 for a 'yard motor + all the other stuff) if the engine is still otherwise giving good service. Since you didn't mention that you were fouling out plugs every other week or whatever it appears that the top end of your motor is OK (valve guides and seals) and that your cylinder leakage is at the rings. The extractors will eliminate the oil contamination of the intake stream ....no more oil in the air cleaner.
How's your oil consumption and "driveability". Is the beast really lacking in power...does is visibly miss at idle..or at any time for that matter.
Try this: Unhook the aircleaner vent to the valve cover and use it as an old style "draft tube" (you may want to put a longer hose on the vent so it can hang down nearer the ground to save the engine bay the mess). That is, let it hang. If your detonation problem clears up in a few miles....voila`.
The extractors are a simple installation that any decent muffler shop can do. It's not rocket science...then again I think it's darn clever.
It all depends on how deep your pockets are at the moment and/or how anul you are about blow-by in an otherwise fully functional engine.
Now that I've quit smoking my doctor informs me that I'll die in much better health!!!