Originally posted by Biff Possible drawbacks?
Dedicated macro lenses are optimized to provide a flat field and minimize aberrations at close focusing distances. "Normal" primes may be a bit dissapointing in that respect if you go to closer working distances than they have been designed for, (using extension tubes and similar). Also bear in mind that you will need a fairly long tube with an 85mm lens to get a decent magnification - some 40mm to get a 1:2 magnification ratio.
You could do an initial try-out of your lens with a simple, homemeade extender made from a cardboard tube or similar?