From my experience with trashy parks, theres about 5 types of areas. (1) clean (2) annoyingly trashed, (3) bad trashed and (4) almost hopelessly trashed, and (5) forget it trashed.
The thing I've found about about small coils in parks is that for deep, old coins bigger coils are a better choice for 1 and 2 and sometimes 3. The small coils are the choice for 3, 4 and 5 but for "3" and "4" I'm lucky to get later date silver and wheat cents, and for "5" I'm just getting clad. On top of that, most trashed areas are hopeless and forget it, leaving only a small fringe "3" area for a small coil to perform. Maybe if I put more time in with the smaller coils I could do better, but it just hasn't clicked for me.
Bottom line, when I'm hunting with a small coil in a trashy park with deep coins, I feel like I'm trying to down an elk through brush with a .22 short. I won't go any smaller than a Minelab 8".
The small coils have been best in non-park areas near foundations, swimming holes, etc. where I can really focus in on high-probability small areas to hunt. Maybe there's ways to do the same in parks but I haven't figured it out yet.
For me, the fact of the matter is that many parks are just too trashy for the small coil to get 6-7" deep where the oldies hang out. Maybe that's why pros like Golddigger sticks with the 10" stock coil and don't fool around with other coils much. I used a WOT since last summer, and if you REALLY like to try a new coil in parks, a WOT makes a better first choice than a small coil. It really does outdo the stock in many situations.