Out of the hundreds of cars I've serviced, I have heard of only one(1)complaint from one(1)owner. A 560SL owned by a guy in Atlanta called back after a service and said he lost 3-5 Mph off his top speed....really. The car idled great! But the speedometer was a needle's width or so lower than it used to be on his favorite stretch of road.
The rationale behind the wider plug gap is that it uses more of the system's "potential" output. This gives a hotter/larger spark even at low speeds(like at idle, when you need it most). The ignition systems on Mercedes since around 1980 (with a few M104 exceptions, but those had a recall on the coils anyway)have more than enough reserve to make use of a larger plug gap without any adverse effects on the components. As far as the "out of the box" spec goes, both Bosch and Mercedes go for a "middle of the road" setting. Back in the 60s, most cars ran well with a .5mm plug gap. That was "spec". Then in 1990 MB decided that all cars should have a 0.8mm plug gap(the same "spec" that was on the Bosch box). The older cars with conventional ignition systems still run better with a 0.5mm plug gap. Think about it, if you "require" Techs to regap plugs(the ones that care do it anyway)by giving different specs for each application(like they should),that's $1 of labor on a $3 part. Life is much easier from the manufacturer's point of view to just screw 'em in.
The bottom line is, for whatever reason....wider plug gaps work. But hey...that's just my opinion(and what I've gathered from empirical data over the last 12 or 13 years).
Regards,
Randy D.