As I have not really found an answer to what effect a blanked of exhaust would have on through exhaust drive and propeller, so I did a Google search and found it is a very technical subject of which a lot of it was beyond me. However it would appear that propeller hub vortex flow can cause up to 5% drag or loss of efficiency. If anyone is interested in this subject a link can be found below to a paper that was written titled “Simulation of propeller hub vortex flow”
"A propeller hub vortex has strong low pressure core in its center. The low pressure of the core causes thrust deduction by pulling the propeller boss cap end. To predict the deduction is important when we design a ship. We have applied numerical simulation method to predict it. Besides, we have measured the propeller thrust in case of non hub vortex condition and hub vortex condition. It is considered that the difference of these thrusts was caused by the hub vortex. The results of the computation and the experiment were compared, and a good agreement was confirmed. The thrust deduction caused by the hub vortex was estimated to be about 5% by both computation and experiment".
For anyone who is racing or has a leisure boat that has a drive that was designed to run through prop exhaust but the exhaust exits straight through the transom, it may be possible to gain a small increase in speed or reduction in fuel consumption by adding a vent to atmosphere on the exhaust port of the drive to reduce propeller hub drag.