brentil - you mention you have to 'work harder' and maybe feels 'less agile'.
These seem like descriptions of maybe steering effort? Is the steering more heavy than your base Solstice was?
If so, this may be on purpose, because the faster you go, the more effort you need especially if you don't have variable steering. The GXP goes faster than the base, and they are making an Opel model which would run on the Autobahn... if you see my thinking.
My impressions are that the steering felt heavier than the base Solstice when I drove a GXP.