[Pharo-project] How important is ANSI Smalltalk / speeding up ASCII/Latin1/Unicode I/O

I have been wondering about ways to speed up character-set conversions, and related I/O as mentioned in the recent email about MultiByteFileStream. However this change might break parts of the ANSI Smalltalk hierarchy - particularly around String. So my question is, how important is ANSI compatibility? Squeak? GNU Smalltalk? VW? VA? On a related question, how common is it to subclass String? Would it be a mess if it were its own little hierarchy of classes? ../Dave