I think the reason that you didn't find this topic is because at 200k miles you are probably 3x(Std Dev) greater than normal life expectancy with your transmission. Either you're blessed or your transmission is days away from catastrophic failure
I think there is a lot of disinformation and superstition around the subject of synthetic lubricants. You could try searching the oil board for this:
http://theoildrop.server101.com/cgi/ultimatebb.cgi. From what I have seen, the members of this board take a more analytical approach rather than just throwing around personal opinions (although there is a fair amount of that)
Definitely, the external filter is a no brainer move; the only way this could contribute to trans failure would be if you hooked up the lines backward
From my knowledge, ATF already has high detergency so I don't think that it would be likely to loosen up a lot of crud since you shouldnt have much in there unless you have been abusing it and not doing any changes (you wouldnt have made it this far if you did).
On the other hand, since you made it this far, why not stay the course and continue using the same type of fluid that got you this far? Syn ATF wont miracularously cure any mechanical problems that you might have; it is just a high performance, heat resistant substitute for normal ATF.
I switched to AMSOil ATF because I live in an area with +100 deg temp most of the summer and I tow a trailer a few times a year and want a little extra margin of safety. However, the cost of the ATF was pretty high ($110) so I also plan to run it for at least 50k. No problems so far (switched at 79k) but I have only run for 2k at this point.
