I really like KISSlicer compared to CURA and Slic3R. I haven't shelled out the cash of Simplify3D yet and I don't plan to in the near future.
If there is a gap between the support material and horizontal surface of the part then the extruded plastic will fall down and sit on the support material and not give you a decent surface. Basically it would be bridging and you'd have a slight buldge downward on that horizontal surface. If there was a way to specify the distance that would be nice as an option.
Here are some options for you:
1. You can convert to HIPS which is a soluble support material in a certain chemical. You need more than one extruder to make this work. One for part material and one for HIPS.
2. You can get a good set of chisels and break off the support (this is what I do).
3. You can print your part at a slight angle so the gap setting will occur (I usually do 20 to 30 degree tilt). I do this for PCBs fit checks I'm printing, you'll use more material, but will have less support material to remove.
4. You can split your part up and print it in pieces and then solvent bond it together. This is what some people do on the MakerGear group.
5. You can try Meshmixer which will create support pillars instead of the typical support grid, then slice in your tool of choice.
ADam