i really dont know how the hard aluminium luggage thing started ... back in the early 90s the germans seemed to get into it, and somehow everyone thought well if they cost that much, it must be the thing to do. Partly because the Germans got into adventure motorcycling at least a decade before the english speaking world did, so what the germans did, the rest adopted.
I think people do the aluminium luggage now because its a de-facto standard, not because anyone seriously has put a lot of thought into "is it a better system" than soft luggage.
The Brothers Vince never allower even the hint of an expensive aluminium pannier anywhere near any of the dozen + bikes from the Terra Circa and Mondo Enduro trips round the world, and while I find their style of travel a little too rustic for my current tastes, it is worth noting that a single 20 litre steel surplus ammunition box was as close as they got to hard luggage. Not surprisingly Lois Pryce similarly used soft luggage on her largely solo rides from Alaska to Tierra del Fuego a few years back and her London - Cape Town 18 months ago. Austin would surely have disowned her if she had used hard panniers.
Hard luggage (maybe it should be called heavy luggage) is there, and people assume its there because its come about thru trial and error, and accumulated wisdom from countless round the world journeys previously made. I think thats actually not the case. Its just there because its there, and everyone adopts it without questioning if it is the best way to go.
So you want to ride around the world? Its automatic that people buy an "adventure" bike then work out how to attach aluminium panniers to it ... cause that what riding around the world is all about, right? Another good example .... (Pete Forwood
http://www.horizonsunlimited.com/forwood/). I've been in touch with Pete since the planning stages for the first leg of his global project back in 1995. Since then he has been almost constantly in the road - mostly solo, but sometimes 2up with his wife Kay. He has been to over 180 countries on the same bike (Including recetly Afghanistan and North Korea). At last count there were only 2 or 3 "countries" left ... isolated islands in the middle of oceans. Pete Forwood is THE DADDY of adventure motorcycling. He doesnt ride an adventure bike. He rides a Harley. And he doesnt have large custom aluminium panniers, he has the bog standard slim factory HD side bags product. Pete is living proof of the meaninglessness of the germanic stereotype of buy a BMW or KTM and stick huge alloy panniers on it to be a "real adventure motorcyclist".
On the plus side for hard luggage, they are more secure, and more easily lockable. But again that depends on whether or not you develop a system to secure your soft luggage more effectively. I have toured on every continent (except antarctica) WITHOUT hard luggage and have not had anything nicked. touch wood.
If you really want to carry large volumes of stuff, dont mind the weight, and have to have the security of aluminium panniers, consider angled stuff as its both stronger and makes paddling in the sand easier.
Vern from this site makes custom aluminium angled panniers as does Al Jesse in the States (
http://www.jesseluggage.com/safari.html) Not sure if either of them are up and running with F800 stuff but I am sure Vern would be able to cobble something together.
Getting back to your original post ... The F800GS will make it round the world just fine without any Touratech add ons. How much you want to spend depends more on how much "bling" you fancy ... think thats why in general on this forum its referred to as "bling". We all like that stuff... but none of it is necessary to ride around the world with ... especially on the F800GS which seems to me to be already a very suitable steed.