I've just watched a youtu.be video on Shimano reels (for some reason no UK videos on them) and it was from Australia as they use the formationed reels in salt water and the Thunnus came out top with the 4000D second also I've checked prices and I can get 2 x 4000d for £266 but the cheapest I can get the 2 x Thunnus for is £490
I totally understand what you are saying about quality but I would probably have to upgrade my rods to appreciate those reels

All 3 models (OC/D/Thunnus) are live bait/spin reels designed for salt water anglers.
We barbel guys don’t really give them much of a hard life at all and being completely honest the upgrades from a OC to a D are largely irrelevant to us really.
Water proof clutch, few grams lighter, and I think they went up a spec on the bearings or gearing.
For a sea spinner these upgrades are worth every penny but to us course guys they don’t really matter a jot.
Moving up to the thunnus and things get a bit nicer again. Lots of bearings put in some nice places to smooth the real up
2 in particular are very very nice. One in the handle and one in the spool to assist the clutch. Again the additional weight savings and higher spec gearing and bearings are more tailored towards salt water guys with cash on the hip.
These are way over specced for barbel anglers as beautiful as they are.
They are designed for screaming zoo creatures out in the middle of the ocean
Barracuda, marlin, small sharks etc. what we do to them is barely taking them above tick over.
Must admit as much as they are overkill i couldn’t use another reel now for my barbel ledger fishing.