I have a 400E myself, I want to make my car as fast as 500E, but modifying a 400E to make it on par with 500E costs a lot, it may be even more expensive than just buying a 500E at the first place. I guess that's why you didn't see too many 400E owners upgraded their differentials.
I will probably buy another MB this year, I want a 560SEC, they are so beautiful, and as fast as 400Es, though their gas mileage isn't good, but I think they have potential to become collector items.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.