Well, its pretty obscure. If Bob's price is right it was more expensive than the 75/4.5 Biogon. It was certainly much, much more expensive than the 75/8 Super Angulon, which covers 181 mm @ f/22. Funny thing is that R'stock made two batches "for stock" several years after they introduced the 75/6.8 Grandagon, which sold for less and seems to have had more coverage.
The few reports -- no formal tests -- I've found on the 80/5.6 Grandy are all favorable. Coverage reports are, well, variable. IIRC, in the post you found Bob said it covers 165 mm.
The VM contradicted itself, as it often does. It first says that both f/5.6 Grandagons (there's also a 58/5.6, Linhof sold them badged Technikon, Graflex sold them for the XL) cover 90 degrees @ f/5.6 and 100 degrees "stopped down even slightly." If true, that works out to 116 mm and 138 mm for the 58, 160 mm and 191 mm for the 80. But and however, a couple of lines down the VM says that the 80 covers 160 mm @ f/5.6 and 160 mm @ f/22. These work out to 90 and 93 degrees respectively. Bob's 165 mm splits the difference.
I have a 58, it is quite a good lens so am looking forward to getting the 80. I'm also doubting my sanity. I want it for 6x12, already have a perfectly fine 80/6.3 Wide Field Ektar thatcovers the format. Not to mention a 75/14 Ser. VIa Perigraphe and 85/6.8 Beryl that both cover. Buying the 80/5.6 was and is hard to justify. Want, not need.