[euromodder]

Bigger tyres will mean bigger frontal area and more drag.
That's going to take away part of the effect of going for taller gearing through taller tyres.

Calculate where your engine rpm will end up.
Then consider wether your engine likes that rpm range or not.
My car won't take a longer gearing without remapping the ECU for more torque at low rpm - as has been done from the factory on the later models.

Bigger tyres are also going to weigh more, and use more energy to get them rolling.
If you do long motorway stretches @ constant speed, fine as far as the tyres go - but the aero effect is going to hurt more in that environment.
If you do much driving around town, the higher mass has to be accelerated often, and they'll likely hurt.