5-6" is probably the limit on getting a reliable bell tone with the Ace, you can get deeper coins, but it won't be a confident bell tone. Also depends where I detect, in dry sand I could probably get deeper, but some parks with mineralised ground can also reduce the depth considerably.
Most tot lots you can always pretty much get through all the bark and into the soil, literally found 100's of 10/5c coins that had worked their way through into the soil. Most $1 & $2 coins get cleaned up before they ever get that far, so if chasing those coins, depth won't be a too bigger issue.
Using the pinpoint button to run in all-metal will pick out deeper targets that won't reliably set off the bell tones, you can usually tell by the response and "shape" of the target whether it is a coin type target or not, but bottle caps and some aluminum will always be dug in the process. Foil and pull tabs are nearly always quite distinctive, very sharp response with a small footprint, and in the case of modern pull tabs, double signals on each swing, depending o orientation.
The Excelerator 10x14 coil, and NEL Tornado should reliably extend depth capabilities by another 2-3 inches, but that might be the time to look at upgrading to a more powerful detector - at least those coils are cheap enough to buy without breaking the bank, they are very popular upgrades for the Ace in both the US and EU.
Best two garrett coils by far are the large DD and sniper coils, with the best depth and stability in mineralised ground from the large DD.