Currently the C++ TZlibTransport hard wires the compression level to default (6). The attached patch allows the compression level (0-9) to be set at construction.
N.B. This patch removes a TZlibTransport constructor signature which has been unimplemented and private since 2010. The sig was left in the code to generate compiler errors if used, however it causes ambiguity with the sole operable constructor and was removed.