There can be a lot of confusion about what features make a detector work, or work better, than the competition. Some comment on using a slower sweep speed, and the Sovereign's are a slow-sweep detector .... but I've owned a few, and used the dinky coil [size=small](I prefer smaller-size coils anyway)[/size] in iron nail infested sites with poor success.
Then there are the other factors that are often discussed such as a detector having a 'quick-response' as well as 'fast recovery,' and that can also enter into the consideration of a detector choice. I have used many two-filter detectors that feature a 'quick-response/fast-recovery' circuitry, such as some newer Bounty Hunters, the Fisher F-75, F-5, Gold Bug Pro and F-19, Teknetics Delta, Gamma, Omega, T2, Euro-Tek Pro and G2.
All of these can respond quickly to a desired target, as long as there isn't a sufficient presence of closely-spaced iron nails or other rejected trash. Of them all, the only one that has worked the best, in a tough iron nail comparison test, is the Fisher F19. It worked better than the G2 I had or my friend's Gold Bug Pro, with all of them using the 5" DD coil or 5X10 DD coil. I use my Nail Board Performance Test to evaluate all detectors, and none of these, with a 5" or 5X10 DD coil, can match the performance I get on an Indian Head 1¢ positioned on the Nail Board, surrounded by the four nails the way some of my preferred detectors can.
Also 'quick-response/fast-recovery' are my Tesoro Bandido II µMAX I use with a 5¾" or 7" Concentric coil, my White's Classic ID that has either a 4½" or 6½" Concentric coil mounted, or my White's MX5 and MXT All-Pro which I also use a 6½" Concentric coil on when hunting dense iron littered sites. Places like logging and mining camps, old abandoned town sites, homesteads, railroad and stage stops and sidings, and many other old-use sites where there's likely to be an abundance of iron debris. All of these models will produce 8-out-of-8 hits on my NBPT, whereas the only one of the above mentioned models that comes close is the new F-19.
I would not waste my time shopping for a 5" coil for the Sovereign. Instead, if you are comfortable with your GB Pro and two coils, use them. Otherwise, I'd suggest shopping for a different detector in order to get improved performance in any dense iron nail site. Since you have the coils now, you might want to consider the Fisher F19. I know it out-hunted the G2 and GB Pro it was pitted against when all of them used the same Sensitivity setting and the Discrimination was set to just barely reject the iron nails.
A local guy bought one brand new, but didn't like the com finish and it didn't have multi-tone like the XLT he had, so he's selling it, unused, for $700, so I am sure you could shop for a clean used or lightly used F19 and mount that 5" DD to it and have improved iron nail performance over what you're using now.
Monte