Has the dimensions of tis mouthpiece changed over the years? The cup depth in particular?

Yes the dimensions have changed - perhaps even intentionally. I've had two Schilke 58s with rather different Cup IDs (but same Throats). Not sure about the Cup depth, though.

Not so sure about the depth — of the sample size of 2 in my life, mine were about the same, I think — but the rims were different shapes. IIRC, the rim on the newer piece (short shank) was not as round as my old (long shank version) 58, though not semi flat or flatter. The bite was about the same, though. Although too small for me now, the old piece is very comfy on the chops.

Also, the blank shape is a little different: less mass up top on the newer pieces.