I think increasing the aspect ratio, 55 to 60, the height of the sidewall, will have more impact on the "softer ride" than a wider tread, 205 to 215.
Wider tread will offer more grip as explained by CapriRacer...
As for the effect increased overall tire size has on speedometer reading (and odometer too)....the difference MAY be beneficial.
When I did so on the Camry (205/65/15 to 215/55/17), the speedo became MORE ACCURATE by 1mph, from being off by +3 to being closer to +2mph at any speed.
...BUT the odometer is now off by -2.5%, i.e. 100 miles on the odometer is really 102-103....not the worst impact.
As CapriRacer stressed, selecting a DIFFERENT tire, brand and model, will have greater impact on the parameters you're interested in pursuing.
You say you don't need "high performance" tires...WRONG. EVERYone needs the best tires for the worst conditions on which they will roll....for me in Florida it's wet road/thunderboomer conditions, i.e.a wet traction tire capable of high degree of water dispersion/high resistance to hydroplaning.
The OEM-spec tire originally on the car would slip off stops in the rain, a common occurrence where water floats on oil drops collected at lights/stop signs, and on freezing rain conditions (okay NOT a common occurrence in Florida, but experienced last fall in Chicago area)...
The new tread, Hankook Ventus V4 ES tires are like glue in the rain under all condition and stresses...I feel amazingly safe on these tires. They are noisier, caused by that grip to the shell-imbedded road surfaces here in Florida, but it's a small price to pay for safety.
Finally, keep in mind the specifications required by Volvo...you really SHOULD keep to the MINIMUM speed and load bearing ratings....many tire dealers will not even mount tires that fall below those ratings...
So as CapriRacer suggested, do your research, HTSS_TR, for a brand/model tire and not just changing tire size.