There is a practical limit. I have never seen mine go above 0.425 either. The typical dose is 0.375 or less. I think the next jump would likely be 0.475 but to do that it would be pressing insulin at over 2.5 U per hour and I am doubtful that would happen.
If you like I can ask Medtronic and see if they know the set upper limit, but i suspect it is 0.425 since I would have rang a bell over that amount many times if I could have.
I have used the pump for about 24 months now and While I do not have every record, I do not see anything higher. Again let me know if you wish me to ask.
Note: I am a Medtronic ambassador. My opinions are my own. They did not pay me to say nice things about Medtronic devices or the company. OK, they sent me a shirt and a cup but even I am more expensive than that.