First up, I didn't do the trip myself, but bought my 800GS from the guy that did.
He rode the whole of Africa - 8500 miles from Morocco to Cape town, and the bike ran stock rims and suspension. He did change the seat for a Touratech one (£300) and I don't blame him, the standard is agony.
He also fited 20mm handlebar risers to improve their position for standing-up riding.
Used Continental TKC80 tyres all the way.
Barkbusters handguards.
Metal mule panniers and crash guards. The pannier setup includes a skinny (but road legal) complete exhaust system which means the left pannier sits as close to the bike as the right.
Bike also had front and rear reservoir protectors, as well as a proper bash plate - also Metal Mule gear.
He swapped the standard chainand sprocket for good quality stuff - the BMW one has a very bad reputation.
I've done 1200 miles in the two months I've owned it, and I'm heading to Morocco in October for three weeks and will not make any mods at all, other than some fresh fork fluid - maybe slightly heavier weight.
This is it somewhere in Africa:
His only problems were the leaking top hose (cured with jubilee clips and should be fixed under warranty) and some new clutch plates at 6000 miles. He reckons it was probably his fault for being too harsh on it off road, though I have heard a few of them suffer clutch failure, especially if the cable is badly adjusted and there's no slack.
I'd happily ride the world on it, and will be tackling the Trans-America's next July.