You can get good test programs and test data file generation from the HP 
web site.

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50460&locale=en_US

Look for 'hptapeperf' and 'hpcreatedata' - the latter can produce specific 
compressibility levels of the data.

That will at least allow you to do definitive and repeatable tests.  I'd 
guess if you set the tape chunk size smaller than the volume of test data, 
you can just see from the logs how many chunks NBU writes, to see if it is 
compressing the data.  Maybe - I'm not sure when it decides to start a new 
chunk if it is allowing for hardware compression.  HP LTT is also a good 
tool for querying the drive status pages.

Definitely worth doing your tests with known brand-new media, as someone 
suggested, once written uncompressed always so written unless reformatted.

William D L Brown