How strong are these rims (stronger than mavic 717's)?

I'd say yes but with the added bonus of also being wider than 717 so support the tyre better (Same internal diameter as Mavic 719 rims). Have 355s on my PA and they have taken all I can throw at them to the extent I haven't used a set of Hoops with Flows I have sitting around 😳

I bought the front wheel off Roger at Wheelpro when they first introduced the 355 on Hoops and he reckoned it would be strong enough for what I wanted but said if it did fail he would re-build…that was well over a year ago and its still straight and true.

I have two pair and at the moment seem to be as strong as my DT5.1's. Bought them with a little bit of nervousness like yourself but I think they are great.