No flame intended and I realize we all view things differently, but, multiple grounds can cause more issues then they fix in the real world. Not that this makes my view correct, but, I am a EE and my specialty is grounding. A single point equipotential ground is always the best since this approach eliminates voltage potentials between conductor sets.
As far as resistance goes, the frame of any car is massive (in conductivity)and the the ohmic value would be in milli-ohms (.001) at any given point on the frame. Impedance which is resistance plus reactance (simplification)could be higher but that would only affect high frequency ac circuits or circuits where the current changes rapidly.
Most problems can be averted by running true pairs of conductors and not relying upon common point grounds, unfortunately this is not the way the oem's set up a car's electrical system. All this is not to say that by luck a second or third ground might clear up a specific issue, but, that is not because a single point ground has a high resistance. If you started from scratch the perfect system would have a common point ground (equipotential) and two conductors per circuit.
If I were to specify an improvement of the vipers grouding system I would do the following:
1. Replace the factory ground wire that runs from the engine to the shielding enclosure under the cowl with a bolted connection. This wire runs to a bolt behind the valve cover on the drivers side (Gen2). Make sure you bolt to the shielding enclosure.
2. Take that same connection point from the shielding enclosure and run a wire down to the frame, once again everything should be bolted and any paint removed. Size is not a big issue, a #12 will work, but a bigger conductor has better strength. I used a #6 flat copper braid on my car.
I have a lot of aftermarket electronics on my car, I always take that same ground point to a terminal block near the device. Once again extending the equipotential ground plane. Equipotential means no voltage variance will exist between any two ground points and the only true way to do this is with single point grounding. Any other way is pure luck.