I haven't beaten them in LoC + Rasetsu yet, but I can say that I've beaten them in LoC mode and my product rank is still 11. Overall, it isn't really a big deal since there's an Item World shop every 10 floors, where you can purchase up to R39 equips and can be legendary as well.
Hmm, I've beaten pringer Z in LoC but not rasetstu and my item rank is 12. Maybe only stuff you buy while in rasetsu mode counts towards ranking it up from 11 to 12? It would make sense because I did a lot of rasetsu item worlding, and I always checked the shops to buy anything with the "new" marker.