If you are going to do the brake swap, I suggest you use the complete 164 stub axle, hub, rotor & caliper assembly between the 140 control arms .... the vented 164 rotors aren't a direct bolt-on swap with the 140 ones, I gather (different hub size ?)

I have this setup on the front of my 144 & the brakes are brilliant.

I run 205/65x15 tyres on mine (on Sirius alloys) and the rolling diameter calculates out as virtually identical to the standard 165R15 tyres - only 2 digits difference in calibration factor on my Terratrip (251 vs. 253) which equates to less than 100m difference over a measured 5km.

I reckon you would could lose a fair bit of "punch" going to the taller diff ratio .... although I must admit that I do like the 5th gear in the Celica box behind mine on the highway, compared to the M40 that was in my previous car :o)

Dave