I am pretty sure the Speedo sensor is on the gearbox, so yes the rear tire size will influence the readout.
I guess it depends what you are trying to achieve with the tire size change. Obviously a wider front tire should provide more grip and less understeer (if this is in fact an issue), but as Wayne pointed out if you get too wide on the OEM rim (6.5″) then the tire tread does not sit flat on the road and you lose some of the advantage. If you stay with the stock rear tire size, you are changing the relative grip levels front/rear, which may not be an issue, but will change the balance of the car. Maintaining the same tire width differential of 30mm (205 vs. 235) should maintain the same balance which would suggest that 215 front and 245 rear is the ideal combination for a next size up tire change. The sidewall ratio will affect both the ride and the tire footprint. Generally a higher numerical sidewall ratio gives a better ride while a lower number puts more tread on the road.