I have smaller trunk bags on both my bikes, but they can foul the saddle and make the bike harder to mount.
I reckon if you need that much capacity you would be better off with a pannier.
Anyhow, it's much better to have weight lower down.
It's in addition to the Ortleib panniers. Have a problem in deciding what to leave out, so take it all when doing long trips in Europe. Spain in a few weeks then Italy.
It's also to replace the rucksack I've been using to get the weight lower down as suggested.
Managed to find a suitable rack on my spares shelves but the brackets are too short (25mm). If I can get longer ones I might be able to set the rack further back.
Topeak have an extra long set (32mm) but seem to have delisted them. Are there any others out there?
Now onto relocating the rear light.