According to the documentaion in the “UDIM Support” chapter, “upscaling” tiles does not affect performance, which probably also means that having all that “empty” space has no effect either:
Technically all you need to import are just the existing images named accordingly to UDIM tiles. The Editor will fill in the blanks without any overhead. However, it is worth knowing that it will go on until matching the highest number of columns and rows defined in file names, i.e. 1007 x 1052 will look like this:
