Virtual Hard Disk Performance

Are there any available benchmarks that approximate the performance difference between virtual disks configured as sparse monolithic vs. sparse split? Perhaps a best practices or guide on how size, usage patterns of guest OS vs. I/O performance against monolithic vs. split configuration?

I suppose the "best possible" case would be pre-allocated monolithic and "defragged" to be contiguous? (Waste of disk space though, eh?)

Thanks for any pointers!