First you can't get an A if you dont sit Higher Tier, regardless of getting A/A*s in your coursework. If you done higher for the final exam then MAYBE you would get an A, but I have to tell you that the Algebra is A LOT harder in higher than intermediate! I found most of the algebra could be done in your head for intermediate, but Higher takes time.
766 is definatly an A, taking that 666 is an A. For an A*, you would need to get AT LEAST 877.
To be honest, my school only puts people in for higher if they plan to do A Level Maths, so unless your planning to, then stay with intermediate. All you need is a C anyway!