The actual Coordinate system is Sphere_Mercator (53004) instead of World_Mercator (54004). Some small gaps between tiles maybe visible at smaller scale if the projection is not set correctly. See below.
Understanding Tile System World Mercator (54004) Sphere Mercator (53004)
Each resource can implement one or more interfaces. For example, an ArcIMS resource can produce Map and perform query, and a SQL resource can store, retrieve image and possibly query. While a File resource can only store and retrieve image.
A particular tile set can mix and match its functionalities. For example, the base map tiles can use SQL server to store tiles, use ArcIMS to make tiles on the fly, and use MS Spatial for data query.
Only Google Map API supports tile size other than 256x256. Larger tile means less server requests but larger download size.
Having map engine render 256 tiles is not very efficient considering data IO, rendering etc.
For most map engine, rendering a 512 x 512, even 1024 x 1024 map takes only slightly longer time than rendering a 256x256 map.
Cut larger tiles then slice into smaller pieces will dramatically decrease the number of map images the map engine has to make. Cut 512 instead of 256 will need only about ¼ of map requests (the boundary may cause small variations)