> So reducing the tyre diameter will not have a noticible decrease in top speed right?

Top speed in the B310 is limited by HP. The stock 80HP A14/A15 have enough power to push the car through the air to about 100mph. The A12, 65HP A14 and 67HP A15 only have enough power to go about 90-94 mph. Changing tyre diameter won't change how much power you have.

The other -- theoretical -- factor is the gearing. You should not rev higher than the engine red line (6000 RPM for a stock engine). With 3.7 gear and stock-diameter tyres, 6000 RPM is ~110 mph. With 3.889 gear it is ~105 mph. But the reality is there is not enough HP to go that fast.

The stock B310 tire diameter is nominally 22.8 inches for stock tire size 155/80R13. The optional tire 175/70R13 is 22.7 inches, practically the same diameter.

To reduce the tyre diameter you could fit 185/60R13 (which are perhaps the shortest 13" tires are currently available). They are 21.6 inches tall. These have a theoretical top speed of 99mph with 3.9 gears -- which sounds ideal for making the power peak at the top speed peak. It will take perhaps 4 miles of track to accelerate up to that speed.

This is all theoretical since no one has actually tested a stock B310 for top speed.

More practical reasons for changing the Final Drive Ratio for a street car (non-racing) have to do with acceleration (lower gears = quicker accleration) or fuel economy (higher gears = increased fuel economy). For a street car, who cares about top speed? Even in most forms of racing the highest possible top speed is irrelevant because there is not miles of road between corners to enable ever reaching such a speed.