Not sure why this is so hard to understand. Take your scenario above. If both dusters were bone stock 14 sec cars, one that was 200 lbs heavier would need about 15 hp more to be as quick as without the added 200 lbs.
Now, if the cars were both 10.5 sec cars, the added 200 lbs would need about 34 more hp to be as quick (i.e more than twice as much). Maybe an easier way to think about it as a percentage of total hp. In other words, how much more hp - in terms of percentage of the total - is needed to offset an additional 100 lbs. That answer would probably be A LOT closer to constant than the actual hp number. Because as already said - the actual hp # is NOT LINEAR! It depends on how much power you're already starting with.
