That's bizarre with the physical expansion slot...

I posted an Excel file on the Roland arranger forum a while ago in which I analyzed what tones are from SRX boards, you might remember. There is a lot from SRX 9, a few other tones from other boards, but definitely not so overwhelmingly many.

Regarding the age of the boards, only SRX 6-9 are really old, they contain the old SR-JV board content from the 90s. SRX 4 an 10 are from 2002-2005, for example (also their samples), and their strings and brass are the closest to software library quality I know of hardware sounds. Of course these sounds are a bit more useful for classical music than for pop music. Anyway, the SRX boards are still traded with high prices on Ebay, because IMO even the older boards have some great samples.
It would be great if you could pick individual samples from various boards for the expansion, but I fear Roland won't offer that for the FA. So technically the MoXF with its flash board is more flexible, but I love the SRX sounds...