Newfiehunter...
I have owned a CZ detector (in one form or another) since the mid 90's and I have had to send only one CZ6a back to Fisher for a new pinpoint button. As you already know the CZ detectors are built like tanks, leading me to believe that you have many more years of good service left in your CZ5. Now, if your like me you can always purchase a spare CZ or two as insurance (or investment) for the coming years.
In response to your question about Fisher's tuning of their CZ3D detectors... I suspect that the Fisher Technicians tune their detectors to a "universal setting" so that they work anywhere in the world... properly with no issues.
I believe I have heard or read somewhere that there were twenty-one steps needed to properly tune a CZ detector. As a result, this would take many man hours and undoubtedly raise the price of a metal detector that has it's sales waning. With today's metal detector market jumping head long into digital technology I think Fisher Labs is trying to manufacture a CZ detector that will please the majority of the people while keeping their production costs down.
Along the lines of my wanting a deep seeking detector... I sent my CZ3D to "Tom D" for tuning and calibration. Tom tuned my CZ3D for peak performance in his low mineralized Florida soil. (My CZ3D is an 1121 series, which was manufactured after the highly sought 1021 series.) Along with Tom tuning my CZ3D for peak performance, he also verified that certain "old" coins which normally read in the medium-tone range on standard CZ detectors... would now correctly read in the hi-tone range (good target) when detected. Tom also calibrated the "nickel" window on my CZ3D so that a larger percentage of nickels (if not all nickels) correctly ID'ed in the nickel range on the CZ meter when the search coil passed over them.
If you have decided against purchasing a CZ3D... Tom also offers the option of tuning your present CZ5 to CZ3D specifications (less the fourth tone found on the CZ3D).
It's been a while since I had my CZ3D worked on by Tom, so I am going to suggest that you contact him directly for his prices. In my opinion... I have found that his prices are reasonable, his turn around time is quick, and his work is worth the price that he charges.
If you need the URL for "Tom D's" web site... send me an email or a PM on this site.
-NEBeachcomber