subtracting out-of-focus images vs. unsharp filter

I am curious about the practical difference in feature/border enhancement 
using the Filter/Unsharp Mask command compared to subtracting a separately 
collected out-of-focus image. I haven't tried collecting these images to 
compare the methods on my own - I was hoping that someone else already 
has. If not, I'll try this out for some brightfield and fluorescent 
images.

On a related note, Camp et al (Nature Medicine 8,1323-1328(2002) - detail in 
supplementary methods) discuss their use of RESA (rapid exponential subtraction 
algorithm) to 
subtract their out-of-focus images. My understanding of how this works is 
that an overall signal to noise ratio is determined for each single 
channel image by dividing the value of the highest intensity pixel by that 
of the lowest intesity pixel (or more likely the 1% and 99% pixels) and 
this value is used to weight the amount of subtraction performed (is this 
similar to th "Mask Weight" value?)

Is my understanding of this method correct? Would the addition of an 
automatically calculated "Mask Weight" value for Unsharp Mask be useful? 
Does such a plugin exist?

Thanks,
Jim Deeds