How big is the print? 3072x 2304 doesn't sound like it would be 96dpi unless the print size is very big. In any case I think that is the limit (300 dpi for a standard slide size)

From the same KB article

Limitations

When you set the ExportBitmapResolution registry value in the registry to change the export resolution of a picture in Microsoft Office PowerPoint 2003, there is a limitation that you must consider. The maximum resolution setting that Office PowerPoint 2003 exports is 3072 pixels measured from the longest edge of the slide. For example, the standard 10" × 7.5" slide would have a maximum effective DPI value of 307. (10 × 307) = 3070. The measurement of 3070 pixels falls within the 3072 limit. Any DPI settings that are higher than 307 for the standard slide would revert to the limit of 3072.Decimal valuePixels (horizontal × vertical)Dots-per-inch (horizontal and vertical)3073072 × 2304307 dpi
Note The limitation of 3072 pixels also applies to saving the slide programmatically. Code that exports the slide at a given pixel measurement will also be limited to 3072 pixels.

This also seems to apply to later versions