I've run fans on 5 different personal bikes over the years. They have all been between 1/8" and 3/8" away from the fins.
If the rubber isolator is in good condition, I never had the fans bump into the fins. However, when the isolator begins to fail, bumping into the fins is your early warning signal that it is starting to go.
In over 200,000 miles of riding with fans I've had two isolators bite the dust, and I've had one motor fail...that motor failure was on a Twin Cam and attributed to vibration, not heat.
I just went out and measured my 2019 M8 TG. It is 3/8" away. I measured my 2012 Twin Cam Softail and it is 1/8" away.
The real danger, or so I've been told by the manufacturers, is that if it is too close, the heat will accelerate failure of the fan motor.
The back of the fans have a little heat shield stuck to them to help protect them from the heat.
Since I know the warnings above, I usually (always when it is fully heat soaked) leave the fans on for 1-4 minutes after shutting the engine down. The heat damage, if it were to occur, happens when the fans are not running and the engine is hot. Thus my reason for leaving them running for a bit after shutting down.
In our testing, we have always had the fans between 1/8" - 3/8" away from the fins...except once, we had them about 3/4" away and they did not cool as well. There is a report on Trike Talk of our fan testing at this
LINK.
However, since I run closer than factory recommended, I am pretty sure the reason that I have not had heat damage to the fan motors is due to running them after shutting the engine down. So, I would not move them closer unless you are willing to do that extra step of letting them run a bit.
OH, and one warning on that...I have forgotten to turn them off and walked away a few times. Once was at a motel, and I did not discover it until the next morning. It did kill the battery (it was already close, but this sealed the deal). Another time was in my bike room for two days. The battery was completely dead, but I charged the battery up, and am still using the same battery 3 years later.
Several times I walked away and came back 2-3 hours later...zero issues, bike started right up.
Kevin