There is a great difference between signal quality and signal strength. Signal quality is the quality of the data being transfered to and from the computer to the router. Signal strength is the strength of the connection to and from the router - the closer you are, the stronger the connection. However, having a strong connection doesnt necessarily mean you have an excellent signal quality.
In your case, however, you have a very good signal strength, but poor signal quality. I am in the same position as you - My router is about 15 ft below me and my computer is under a desk with low clearance. However, I do not have much performance issues.
Do you have any cordless phones around the house? Cordless phones degrade the signal quality dramatically even though it is not in use. These kinds of phones are operating about the same radio frequency as the router so there is very strong interference - think of it as running through an obstacle course. Any device that emits radio waves will impede signal quality.
My solution to you is to set the router to transmit at either channel 1, 6, or 11 (11 is best) and increase the antenna transmit power (at the cost of noise) on both the router and computer w/ wifi device.