15-85 +12mm results:


85mm, farthest focussing is about waist-high to the ground.


15mm, focusses perfectly on the waterdrops on the front of my lens, put there by the dew on the grass i was trying to focus on


.


and finally, nifty fifty 1.8.


farthest focussing with a 12mm tube is about 20-30cm.


.


.


so you don't have the same lens as me, but the trend is clear: ie, it's not worth it unless you can find a 1-2mm extension tube.


just
buy yourself a good sensor cleaning kit and get used to it (or just
forego being able to take really macro shots without changing a lens)