-- Last Updated: Sep-30-12 8:44 AM EST --
Do not invest in another boat until you have gotten work in those classes to learn how to use this one. As others have said, it should fit fine once you understand how it is supposed to work.
Kudzu's suggestion to move the seat back is a good first start, though that needs to be done with regard to the correct location for the thigh braces.
As to the adjustments you are considering -
Raising the seat in its entirety is not what people really meant. The adjustment that you may want to add at some point, but not today, is to fill in the lowest part of the seat with minicell or change its angle so it is flatter. This in not the same as raising it up.
A 16 inch tall deck is a very tall deck - way taller than should make a proper fit for you especially once you get the thigh braces over your thighs rather than your knees. If you got by the first round of lessons in it you would find a need to get another boat when you got to trying to roll it. You are buying yourself a terrific backache if the deck is too tall. We know someone who finds the Tempest 165 to be quite comfortable and is at least four inches taller than you. You are just at a point where you don't fully understand fit because you don't understand how to use the control surfaces of the boat. You can't separate the two from each other.
AS others have said, you have found a great deal on a very good boat to get you started in sea kayaking. The Tempest will support your learning what you need to know and be a boat to keep around for guests if you want to go to something more specialized later. Get some advice on fitting it out when you get into that class.