As a T1, I have experimented with Metformin. And it makes a big difference. I don't have significant insulin resistance. And the BG reduction Metformin had on me is caused by a different mechanism. Essentially, it is the inhibition of hepatic glucose. Metformin reduces the production of glycogen by the aplha cells, which in turn releases glycogen into the blood stream. This pushes up blood sugar, especially after meals.
So, yes Metformin can help T1s contain the DP effect and post prandial blood glucose excursions. But it isn't usually prescribed for T1s. I stopped using it mainly because it was complicating my insulin/carb ratios. I also had a few hypos after startingh a regular exercise program. I figured that I could do without it.
