Hi Mileme,
My 2128 is rated at 280gph (but it will be less after losses) and I still would have liked a little more flow. The surface swells a bit right above the return but it would take a lot of flow to actually splash over the edge after first travelling through 12 inches of water. I keep my water level about 1/2 inch from the top and never had a problem. The only time you would experience overflow is if your water level was even higher or your bulkhead really close to the edge (like closer than two inches).
If I could find a filter with an output of 350gph I would use it - and I still wouldn't expect a problem with disturbing my sand because the flow is directed straight up. After hitting the surface, the current is a gentle, random turbulance by the time is gets to the bottom. HTH Good luck.
- Chad