The Omega 8500 is good mid-range detector and a logical incremental upgrade to the 4000, but other than GB adjustment (which may or may not be needed at your site) and unless you are getting the 11" DD coil vs. the std 10" coaxial (which ups the total price another $50 - $100) don't expect a huge increase in performance vs. the very capable 4000 with its 8" concentric (the coils should be swappable between the two detectors provided you have the newer screw on connector on your 4000 - my point is difference in coil performance between the 8" concentric vs. the 10" coaxial is increnental vs. the 8" and the 11" DD coil that I might consider paying the extra $$ for the 11" DD if the cost delta was less than the delta between the 11" and 10" coils if you bought them standalone, that way you will maximize the performance difference between your 4000 and the 8500). Also, I would get some swing HOURS in with the 8500 before hitting the site so you are not learning the 8500 on the site you only have one chance to hit. If you don't have the opportunity to get time on the 8500 before your site visit, bring your 4000 along as a backup and don't be adverse to pulling it out of the car if you are getting no joy on the 8500 (you shoukd bring the 4000 as a backup, regardless). Also, before you spring for a new 8500 look for any trusted, gently used 8500 or even 8000's out there (that's how I obtained my 8500 for cheap). The savings you get may be enough to go towards buying the 11" DD or a smaller third specialty/sniper accessory coil that you could use on either detector. Finally, you might ask for opinions as to whether it makes sense to consider a Tek T2 Classic which can be bought at $499 new and may get you an even bigger bump in performance vs. the 8500 though you cannot swap coils between the T2 and Delta because they operate at different frequencies (~7 khz for Tek greek series vs. ~14khz for the T2). Good luck.
