They are all too good to be true. The only time you need or want a performance chip is if you are putting it in a modified engine or one that is high performance. The 3800 Series II is a great engine but it is not high performance. The only thing that will happen is you get worse gas mileage and the little performance you will gain will just be due to the chip telling the cpu to send more gas in faster, this will only lessen the life of your engine.
Best eco mods are driving habits, higher pressure in tires, anything aerodynamic you can do to the body, and warm air intake systems. The Buick 3800 have taller gear ratios so they naturally get better gas mileage than the Bonnes.