I finally sit down and do some calculation, to my surprise, the size of the solar disc will be only around 750x750 pixels at 400mm focal length!
Therefore, even if I get a 2x tele-extender to use with a 200mm f/2.8 (i.e. 400mm f/5.6), the size will be just similar to my DMK + Borg 45ED II combination.
When I asked in a local forum, a shadow chaser told me that we will only need 4-5s of exposure at ISO 100 even at f/11 to capture the fainest part of the corona. Therefore, if I use ISO 800 which is still rather clean with today's technology, the exposure is well manageable even at f/14.4 of my Borg 45ED II with 2x barlows! That will be 650mm which is quite pleasing. With such short exposure, even a roughly polar aligned mount will do the job.
So it seems like that a 200mm f/2.8 is not really necessary for this trip at all.