Hi Oliver,
I'm a big advocate of having extra spools with different line classes to cover all the different scenarios I come across out on the water. For my Saltiga 6500H, I've got 2 x 6500 spools (1 with 300m of avani GT pe8 and the other with around 450-500m of avani casting pe5) and an RCS7000 with around 250-270m of avani gt pe10.
Daiwa should be able to sell you a spare spool for the dogfight so you can have the best of both worlds.
I think it's a waste having 400m of pe8 for GT fishing though because you'll only ever use the top half and you're stuck casting with an unnecessarily heavy reel all day long. I understand that it's needed for the big blue fin but this is where the spare spools come into their own.
Maybe a spare spool for the 5500 to run about 250m of pe8 which would be perfect as an all day pe8 GT casting weapon in Musandam? Don't let the 15kg max drag concern you as it's definitely adequate for pe8 work.