So, how much you notice a die size difference of one step is going to depend a lot on what sort of actions you use. For ease of math and because I can’t do complex AnyDice functions, I’m going to use spreads of 3d6 against 3d10.
Overcomes: MindWander already covered this, but rolling 3d8 you’re looking at a 31% chance of a major twist, a 64.7% chance of a minor twist, and only a 4.3% chance of no twist. If you can use a Principle to use your Max die, you are instead looking at only a 5.3% chance of a major twist, a 61.7% chance of a minor twist, and a 33% chance of no twist.
If you have three d10s, your chances change a lot. Your chances of a major twist drop to 21.6%, with a 56.8% of a minor twist and a 21.6% chance of no twist at all. Get your Principle aligned and you drop to a mere 2.7% chance of a major twist, 31.6% chance of a minor twist, and a jump to a whopping 65.7% chance of no twist.
So, over the course of ten Overcomes using your Mid die and ten Overcomes using your Max die, a character with straight d8s is netting around 3-4 Major and 12-13 Minor twists, while a character with straight d10s is netting only 2 Major and 9 Minor twists. That’s a pretty big difference for a single die step.
Boosts and Hinders: In the case of Boosts and Hinders, I’m just going to quickly divide by Min, Mid, and Max and give you average results, because there’s a lot of math behind it.
On average, three d8s gives you a bonus of 1.25 with your Min, 1.73 with your Mid, and 2.27 with your Max. Three d10s gives you a bonus of 1.37 with your Min, 2 with your Mid, and 2.6 with your Max. So over the course of rolling with five each of Min, Mid, and Max, you’re looking at a rough total of 26 points of bonuses and penalties with your d8s, and 30 with your d10s. Not a huge difference, and probably low enough to vanish into the static if you’re not looking for it, but with the potential to mess things up.