Will a taller tyre affect suspension geometry?

It won't change the suspension geometry. I see no trade off except for possible tire rubbing when making sharp turns. Personally I would have looked for 40 series tires to keep the diameter closer to stock.

235 is a bit wide for these chariots. 215-225 is the Goldilocks zone used even now. If increasing ride height to start to approach OB clearance was the goal I would've used 225/50R17, which is just shorter than the 225/55R17 size used on the OBs. Your 235/45 should work just fine, but with a bit increased drag and some slight loss in fuel efficiency. You might like the perceived increased grip, but you don't have enough power to take advantage of the +10mm tread. But they'll fit fine. And yes, use a GPS to recalibrate the speedo if you want, but note that Subaru always erred up in calibration, so you might not see as much of a negative speed error as tire/wheel calculations would predict. It might be only about -2 or 3 mph at highway speeds....

Ed, the calcs may be correct, but in practice the delta will be closer to plus 4-5%, and not bother anything.