I don't believe that there really is a need to upgrade the brakes on a 400E. Aside from the fact that the brakes on that car are already quite good, upgrading the brakes on that car is not as simple as bolting new calipers on.
Read my previous post if you want the nitty gritty details, but in a nutshell, to go to 500E or 500/600SL brakes EVERYTHING from the steering knuckle out would need to be changed. Period.
I looked into upgrading my 16v brakes to 500E brakes. The 16v, as some of you may not be aware, uses the braking system from the W124 300E. In order to accomodate the 500E brakes, I would have had to shell out more money than it's worth.
Unless you plan to race your car, 400E brakes should be ample.
I don't plan on racing my car, but if I come across a set of 400E/300CE brakes I'd upgrade them because I prefer the fixed caliper design to the stock floating caliper. but that's just me.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights