You are correct. When you have multiple MAPs connecting to a RAP they all share one channel thus your total througput is divided between all MAPs. Now the "halfening" effect is happening because you have the same radio receiving a packet from downstream MAP and transmitting the same packet to the upstream MAP. Since this is happening on the same channel all participating cells have to shutdown and listen while a packet gets retranslated (and I am not even mentioning the ack for each packet!)
In your question I believe you are using two words bandwidth and througput but they are different. We usually use Data Rate for the transmission speed per packet. That speed depends on a number of factors: noise, signal-to-noise ratio, amount of possible MIMO streams and other RF parameters. Data Rate dictates the maximum speed for your transmission. If you plug your numbers in the spreadsheet that Scott provided you will get your data rates.
Throughput is how much data you can actually push. It is common to anticipate a throughput to be about 1/2 of your data rate but that is only the begining. There is a need to factor retries, amount of clients/MAPs/hops your system has, RF colisions, retries etc etc. Solid estimates for throughput come out of a site survey. Actual numbers often are not available until after post-implementation survey.
