Not any moreso than doubling the number of turns it takes.
i.e.
Player A rolls 1d6 : An average roll (3.5) would take 6 rolls (5.71) to total 20
Player B rolls 2d6 : An average roll (7) would take 3 rolls (2.86) to total 20
It will still take an average of 6 total dice to reach the minimum value of 20, with a minimum of 4 dice to total that value.
In other words; how is it any different than player A rolling four separate 1's while player B rolls two 2's. I think two dice (2d6) makes an "average" shot more reasonable, and thus "par" easier to calculate. In this case, 2d6 will average 3 turns to complete the hole, thus giving the "par" for the hole. In golf, par is considered "average" when calculating difficulties for holes.
http://www.thefreedictionary.com/par
Im not doubting your maths, or your definitions.
But you have to take into account that this is a game based on interactions with other people and thier balls.
If I was playing it solo, i would probably do it your way.
But its multiplayer and thus the interactions come with bouncing balls off each other.
I fully expect each hole to take 8 to 10 shots barring good rolls. I cant say its a Par 8. The only link between the different holes I got to go on is how many boards they are made up of, thus thats what I use for Par.
If you take 6 shots on this, there is nothing in the game to say you are 3 over par.
Its something I am looking at, but not solved yet. But rolling two dice didnt work when i tried it out with my friends.