I don't believe it'll affect anything except maybe the speedometer. I don't know how that car collects speed data but I'd imagine it uses a cable tied to the tranny, or an electronic pickup but still in the tranny. Point being that if it reads from the tranny it will then read low by that same % difference in your gears. If it collects it at a wheel then it'll read normally and you're good to go.
Other than that the eng/trans will simply see a greater load at all times but will run fine. So I'd imagine city mpg will end up being slightly worse. Freeway mpg may not change but if it is better it won't be by much. So I'm not sure what the point of changing the gearing like that would be... Not only will you gain little to nothing, but you will lose that % of torque and you don't have much of it to begin with.
If you're getting a used Torsion diff then I do understand that, but if so I'd swap the old gears onto it, or even lower ones. I doubt any E430's have a Torsion. If your gears are just shot and this 430 diff is a cheap replacement, then I guess go for it, but if it were me I'd look harder and find one with the same or lower gearing that I had. If your car is all wheel drive then obviously you have to swap both gear sets which would be super not worth it.
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