That's what I did on my Vectra as well.I made the change to 225/45/17 years ago as tyre prices tend to be cheaper as there's more choice. The speed difference is very small, from memory its something like if the speedo says 60 i'm actually doing 58mph.
That's what I'm running but when I did a quick bit of research before I looked in owners manual it threw up 215/50/17 as the size which made me think. Still undecided about a spare. Never been a fan of tyre weld etc as most tyre places hate doing tyres if you've used it. With ultraseal and slime at least it stays in a liquid form and supposedly gives dynamic balancing to your wheel. I did run slime in the wheels on my harley but the type designed for tubed wheels.