A way of sharpening is to add multiple high-pass layers, progressively reducing the radius and increasing the opacity.
Attached is a macro to do this, including the ability to tweak adjustments. Please try it out and let us know here if you find any better adjustments.
Apply it to a duplicate layer so you can switch the adjusted layer on and off to see the difference.
This seems to be quite labour-intensive, including as it readjusts when you zoom in. It can hence be a good idea to merge the effect into one layer (turn off the original layer and layer/merge visible to do this).
You can also try adjusting the opacity of the adjusted layer and even play with blend mode (eg. try Luminosity).
Dave's 6-Layer High Pass Sharpening.afmacro