*Deep Breath, Touches Wood* They've been on a year now and no issues at all!.
But 215/40/17 is further away at -1.95% with 215/45/17 at 1.46% difference, also a wanted to keep the sidewall height as near or higher to lessen the crashy ride round here.
Speedo reads +1.5 km/h different I believe from the tools I used online to check, your figures look spot on there ffoxy.
I know its better to under-read, but speedos mainly under-read anyway so the bigger tyre just brings the accuracy up.