I don't think that the size of the rom is the problem here. First of all my Xbox has a 128 MB RAM upgrade and I also don't think that the result would be black sprites here and there. If I remember correctly the game uses some special graphic effects when the sprites become black (light effects) so maybe this can be fixed with the settings and/or the correct Rice plugin or the effect is not yet emulated with the Surreal64.
Did ever anyone fixed this black sprites problem with the Surreal64?