There is about 5kg weight difference in my smallest and largest fuel tanks, not to mention the fuel that sits in them. I found last season to be effective on an 80L tank, this is generally enough fuel to run 18 laps stints. If you've got a 150L tank that weighs the same as a 100L tank then that's awesome.
I remember designing 50 L, 80 L, 100L, 130L & 150 L tanks; which were within a few Kgs of each other so I figure losing about .500 seconds per lap in qualifying trim for more open strategy that it was worth the little bit of time lost.
Remember most tracks take 20 secs - 25 secs per stop so the longer you are on the track the better it is in the long run, So a 1-2 stopper is always quicker then 3-4 stopper at most tracks. This might be different if you have very good tires since tires do make a huge difference but for the past two season I have been using the worse tires meaning they lasted maybe 6 laps if luck. So my cars were already at their average worse speeds after 6 laps meaning it made sense to do long stints and limited stops.
I have much better tires this season so I am going to have to play around a bit but I think the difference is only going to be that a 2-3 stopper is quicker then 1-2 stopper with a bad supplier. That isn't that much of a difference in most cases.