I have checked that:

track->pan is only set in two ways:
1. directly, in that case it is always set to multiple of 1000s:
imuse_track.cpp:
129:	track->pan = pan * 1000;
223:	changeTrack->pan = pan * 1000;

2. or void Imuse::callback():
250:						track->pan += track->panFadeStep;
252:							track->pan = track->panFadeDest;
258:						track->pan += track->panFadeStep;
260:							track->pan = track->panFadeDest;
panFadeDest is also only set to multiples of 1000.

So that value, which have special handling (6400 in the old code) doesn't seem to be possible when directly setting the pan or when the pan fading reaching panFadeDest. Sure, there is a remote change, that during fading of the pan 6400 is hit. But still: if there is an intention to fade from one side to the other, why should somewhere around 10% of the left range the pan actually be set via mixer to be fully balanced?

That's why I'm believe that 6400 is some kind of a typo.

An additional hint is, that the lua code frim GRIM and EMI uses 64 as default value when setting the pan (e.g. via ImSetParam).

I haven't find any evidence, why a value of 6400 should be handled special. But 64000 would correspond with a balance of 0 (fully balanced).