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I have 215/55/17 tyres with 'lowered' suspension the previous owner had setup. The 17'' wheels are an improvement over 15" in getting over earthquake lumps in christchurch etc but I feel like going to say 65% from 55 would improve the ride a little. I also plan to return to factory ride height.
I'm fairly certain this minor change in tyre circumference will physically fit ok, especially if I raise the ride height, and my speedo will read slightly out, but are there any other reasons why I should re-think this plan?

Yes, you definitely need to have a re-think.
The reason the car has 45 profile tyres is due to the 17 inch rims. It's likely the same car with 16 inch wheels may have 55 profile tyres, or with 15 inch wheels, 65 profile tyres. This is to keep the overall diameter approximately the same.
Yes, lower profile tyres will give a harder ride, so if you plan to return the ride height to standard it may pay to fit smaller wheels with higher profile tyres to eliminate the hashness. You can't just fit higher tyres on the 17 inch rims.
As an aside, the 17s will not be an improvement over the 15s for getting over bumps in the road, as the total diameter should be largely unchanged.