G'day
From what I have seen over the years the only detectors that would benefit realistically form mods is the early Sd2000 and maybe the Sd2100 as well, the problem with the later machines is that in some cases they can become noisy and that makes them harder to use, you often see modified detectors for sale on ebay and gumtree and people struggling to recoup their costs of what the machine is worth as stock standard and the added costs of the mods that have been done, mostly they lose money on the machine for two reasons, firstly people wonder if the mod is that good why would they be selling it and become suspicious that there may be a problem with it, also many people know that if there is an issue with the detector minelab wont repair it so it can only be sent back to the person who did the mods so will want to avoid it.
Best thing to do is to try different types of coils, a lot of people are saying that they are getting improvements to the performance using the new flat wound coils such as the nuggetfinder evolution and the coiltek elite, maybe this is a good place to start if you want to get some added performance from your detector, and it is obviously a cheaper option with no down side like having it modded, even if you still consider having it done see if you can get in touch with someone else (bound to be someone on this forum) that has had their machine modded and ask the question of whether or not the improvements were worth the cost or what issues they have encountered with it?
cheers
stayyerAU