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 the worst case. The rate for
* currently supported encoding pairs are within 3 (SJIS JIS X0201 half width
* kanna -> UTF8 is the worst case). So "4" should be enough for the moment.
*
* Note that this is not the same as the maximum character width in 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