I have twin Elkamet tanks on my F650 (aka wrongly as Touratech tanks as they sell them but they are made by Elkamet)
Both tanks have a venting pipe to the air. No vented locks or caps- Just good old fashioned tube open to the air-
IMHO, these are much tougher than OE tanks but no weight saving there
I took a dive at 60mph and the bike slid on both sides
on the tanks- they got a bit like 'grated cheddar' where the rub was but did not puncture.
Aside from the added range the other benefits are one of convenience and more importantly their central location and proper weight balance -
and as for range...oh yes, a hair over 1000kms! The max I ever 'pushed' these were in Africa and 815Kms without finding any fuel!