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You'll usually introduce some speedo error by changing wheel sizes, but not as much as you'd think. The extra wheel radius should be offset by the lower profile tyres. It's the overall rolling radius that counts.
A Google search for "rolling radius calculator" throws up a few sites. I haven't tried any lately, but give this one a go for starters. You'll need to know what size tyres were on your old wheels as well as the tyres on the new ones.
Inconsistencies in the road will certainly transmit much more readily through bigger wheels and lower profile tyres. That's probably the main argument against going for bigger wheels. If the vibration is severe, then it's possible that you've lost a balancing weight since having the wheels fitted. Sometimes they just don't stay put.
Bigger alloys are also far less forgiving of being kerbed, and that may also lead to vibration. If you (or another driver in your car) has managed to whack them against something, there's the possibility that the tracking's out tracking out. You/they may even have warped a wheel.
Take it in for re-balancing and 4 wheel laser aligment. That should identify some of the more likely causes.
Currently in Horatio's garage:
2010 Laguna III Initiale.