I just finished my conversion to an E3D hotend, and it is glorious... everything works great... You need a new auto bed leveling part to print, and a new fan shroud (because the E3d is 10mm longer than the jhead stock hotend). everything is perfect.
.4 mm nozzle now (which i like) and printing like a CHAMP.
With FDM, there is no real need to go below .1mm layer height, its almost not even noticeable... with GOOD printers its not even noticeable at .2mm.