I myself would have no problem reducing the load some, I would first compare it to other regular shot loads using the same powder and decide from there. (This is something I feel safe doing, I am not recommending it to anyone else.)
I have made roundball and buckshot loads just by using regular shot data and and round balls or buckshot instead of regular small shot. (This is something I feel safe doing, I am not recommending it to anyone else.)
You didn't say what gauge you were asking about, but I assume it's 12ga if it is I know there are loads listed for 1 1/8oz shot using Unique at around 1200 to 1250 fps and they use a lot less powder then the 1 oz slug loads that were listed, so I would have no problem using that data in place of the larger charge. (This is something I feel safe doing, I am not recommending it to anyone else.)
But in the end your the one shooting them so you have to be the one to decide what you think is safe and what is not.
Michael Grace