i don't think that your 1985 500sl set-up is too much different from my 1986 560sel or my 1987 560sec.
the impedance in the high voltage ignition circuit is intended to attenuate, if not suppress, rfi[radio frequency interference]/emi[electro-magnetic interference] that is emitted by the high voltage arcing at the spark gap.
the best way to achieve this is to fix a resistor within the spark plug as close to the spark gap as possible. if my memory serves me accurately, the sae standard calls for a nominal 4,000 - 7,000 ohms to achieve sufficient attenuation.
however, different spark plug manufacturers use different resistor methodologies to accomplish this impedance. some methods are better and more durable than others. and it used to be the case that resistor-style spark plugs were more expensive than non-resistor spark plugs.
for a vehicle of your vintage, i would hazard the opinion that the only device adversely affected by rfi/emi would be the radio. if you don't hear ignition noise on your radio while the engine is running, i would hold that you don't have an rfi/emi attentuation/suppression shortfall.
but now that i am on this topic, and it is a favorite one of mine, i care to tell you that daimler-benz ag and robert bosch gmbh developed spark plug boots that really suppressed the emission of rfi from the spark plug. the boots were molded of a hard thermoset plastic[phenolic, i think] with a wire wound resistor embedded in line between the connector to the spark plug terminal post and the wood thread screw upon which the non-resistor, 19 strand copper conductor 7mm ignition cable would be secured. this fairly stable resistor, immediately upstream of all potential arcing points involving the spark plug[spark gap, connection to terminal post] sufficiently attenuated any rfi/emi that could travel into a high voltage wire that could then function as a transmitting antenna radiating lots of rfi/emi into the engine compartment and beyond.
to complete the suppression, the phenolic insulation was jacketed with a galvanized steel shroud which fit tightly to the spark plug hex. this effectively shielded the spark plug preventing any rfi/emi that would be emitted from the ceramic insulator from escaping.
in my experience, this arrangement was one of the finest ever developed at the time. and as i said before, if you aren't hearing ignition noise over your radio, you are not having a problem caused by rfi/emi.
however, you could have other electrical deficiencies with your ignition components. and they can easily be checked out using a simple volt ohmmeter.
if you think ignition deficiencies could be your problem, i would do these things...
1. pull spark plugs. check them for continuity. certain spark plug designs can have no continuity quite routinely.
2. check all secondary leads from spark plug termination to distributor for continuity.
3. examine distributor cap and rotor from the inside. if you are still running the originals, i would replace them.
4. check the ignition coil for its adherence to electrical specs.
5. have the engine scoped. distributor gears wear. you might find that your timing and other aspects of ignition[dwell time, etc] are off of the mark.
lastly, you can have too much impedance[resistance] in the secondary. so you want to know how much you actually have. because though too much will attenuate rfi, it will also reduce the amount of energy in the spark that arrives at the spark gap. if you pay attention to your fuel consumption per miles traveled, that should give you a clue as to how successful your in cylinder combustion has been. if your mpg experiences have not been deteriorating, then i would say that you have not weakened your spark energy deleteriously with secondary ignition circuit impedances.
go put a scope on your engine is my end of post reply.