Hi Gorgo.
What I have found when going too short on paddle length is damage to the rails of the SUP. The edge of the blade can strike the rails more often.
My board is only 7ft for foiling.
I have experienced this more so when foiling due to trying to turn the board around in time before catching a wave. Trying much harder than normal. And also just as the board lifts higher than the water level I can miss the blade digging into the water as I am paddling hard which it then bangs hard against the rail. Usually happens on the end of my turn going forward.
This happens mainly from my shorter paddle which is as high as my forehead.
My longer paddle is about 4 to 6 inches above my head. So therefore it is mainly the round paddle shaft that hits the rails of my board.
In my view I would only take 2 inches off your paddle length then maybe only 1 inch if needed after that. It all depends on your style. Some dig deeper than others.
Cheers
Wil