A bit late, but you should be aware; the PSR-S packs purchased prior to... I think YEM 2.4? *did* work on the Genos (except the s670, a lot of those voices are greyed out). So if you purchased the packs before they went free, even if they require an email login for verification, they will generally work on the Genos. A good chunk will work on T5 even.
*However*, when they started giving out the free packs, the new ones are locked to the target kb model only. I don't know of any way around that. So yes, you should call Yamaha Music Soft and they can tell you what will and won't work.
Opinions:
The Church & Christmas pack was essential for PSRs series, as none of them had good solo strings (the 1 exception now is the PSRsx900), plus again, other than the sx900, the internal pipe organ wasn't sampled, so the C&C pack ones were a huge improvement. For the Genos, not as essential since it already has great built in organs, strings, and choirs.
Yes it depends on what kind of music you want to play anyway; now that the packs are free but KB locked, you might as well try the available Genos ones. But even then, space limitations won't allow you to fit everything on there. There are now 3 Brazil packs, for example; if you want to play Brazillian music, then yes install all 3!
Personally, since the Genos lacks the built in Indian, Chinese, and Middle Eastern Kits that the PSRs series includes, those would be the kinds of packs I would install (but if you don't play any of that music, then maybe just the drum kits, not *all* the content). I also really like Greetings from Italy with the Solo soprano voices, and I'd top it off with Latin and Brazil packs.
Also, the PSR's organ pack was great for the PSR's that didn't have any sampled pipe organs, but since the T5 and Genos both have "organ world", those voices become redundant.
Mark