What you have read online is 100% correct...it's not just Campbell Hausfield...it's most compressor companies (not to offend, but especially compressor manufacturer's that aren't on the high end). The 11 CFM that the manufacturer claims is what the pump produces at maximum RPM.
As far as what your producing with the filters...I don't know...we can guess...but without knowing exactly what your compressor is putting out, (manufacturer's have been known to embellish on CFM output as well). If the compressor manufacturer is correct in claiming 11 CFM....you can flow another 59 CFM through your filtration system without any problem. Filters need to withstand much more CFM than the compressor's they are hooked up to. If your concern is that the filtration system is going to restrict your air lines...and from what I'm reading it sounds like that is your concern...going from 3/4" to a 1/2" filtration system...you will not notice it.
You are wise not to run your sandblaster for very long...if you did, many problems will occur. The compressor will over heat, causing warpage of the compressor head, condensation and possible oil contamination in the air that you are now trying to filter and in turn reducing the life of your filtration system, your compressor, your air lines and your bank account. So in short...don't run your compressor to the point that it overheats...it costs lots of money.
I hope this helps you...I know it can be confusing. I will say it again, going from 3/4" to 1/2"...you will not notice it when your sandblasting.