What is the maximum encoding-conversion growth rate, anyway?

I just rearranged the code in mbutils.c a little bit to make it more
robust if conversion of an over-length string is attempted, and noted
this comment:

/** When converting strings between different encodings, we assume that space* for converted result is 4-to-1 growth in
theworst case. The rate for* currently supported encoding pairs are within 3 (SJIS JIS X0201 half width* kanna -> UTF8
isthe worst case).  So "4" should be enough for the moment.** Note that this is not the same as the maximum character
widthin any* particular encoding.*/
 
#define MAX_CONVERSION_GROWTH  4

It strikes me that this is overly pessimistic, since we do not support
5- or 6-byte UTF8 characters, and AFAICS there are no 1-byte characters
in any supported encoding that require 4 bytes in another.  Could we
reduce the multiplier to 3?  Or even 2?  This has a direct impact on the
longest COPY lines we can support, so I'd like it not to be larger than
necessary.
        regards, tom lane