Personally, I'd go with something like mud jacking- the shim thing probably is ok, but that kind of thing bothers me.   
 Much in the way that putting sugar packets or folded cardboard under a wobbly table leg bothers me.  
 
 If you do go with the mud jack, let us know how it goes- both cost wise, and effectiveness.  I'm very interested especially if they can lift the slab with that full spa on it.