Bose-x
I cant see the savings between FL80 and FL120 in terms of fuel burn being "massive" as you put it when you have taken into account the extra fuel burnt in the climb and the reduced TAS.
I can't do the sums for you as I don't know what your airframe/engine cost per hour is but I do know that with the increase in fuel costs the airlines are reducing cruise speeds to save fuel this increases the "airframe" costs but the bean counters have done the sums and found this to be the cheapest way to fly the aircraft from A to B and that points the disussion in your direction.
So please take a look at a 2 hour flight at FL80 and the same flight at FL120 and tell me what the total flying time and total fuel burn is, we also need to know your "dry" hourly costs. Only then will we get a picture for your aircraft, I know that for my aircraft FL80 is about the best, you could well be totaly correct for your type of aircraft.