I'm not 100% sure about the 300CE calipers. I've not had the chance to verify the information, so at this point it's second hand at best. However it is possible that the upgraded master cylinder is a result of the increased speeds of which the 400E is capable.
I would recommend to anyone who decided to significantly upgade their brakes to take a close look at their booster and master cylinder as well. IMHO you can't go wrong by upgrading them at the same time.
'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