1st question - not sure there is much you can do there
2nd: To go from 57 to 60 mph shift, you'll need 5.3% bigger circumference wheels. The guidelines say you should keep the size difference to 3%, but I think 5.5% should be OK. Between the stock options 205 55 16 / 225 45 17 / 225 40 18 there is less than 1% variance. You'd have to go to 225 50 18 to make a significant difference.
To change gears in not quite as easy as you'd think as the gears are rather tightly mated on their shafts. Now the difference between the 2.5 , 3.2 (GTA / GT) box , the 3.0 GTV box is ONLY in the final drive - still you'll need the whole secondary shaft and the crown gear (see point 1). There is a 7% increase between the drives (for the ease of remembering - the 2.5 box red-lines in 6th at 160mph, the 3.2 box at 175mph, the 3.0 box at 186mph). The Diesel box has the same internal config, just another bell housing. It has longer ratio jumps with 1st the same as all the rest, but 2nd, 3rd, etc are incrementally longer and it comes with the 3.0 GTV final drive - that said it should hypothetically red-line at 250mph if put in a petrol box.
All said: if 0-60 without shifting is your aim, then fit the 3.0 GTV box. If your aim is 1/4 mile times, then stay small ratio - 2.5 box. If you are aiming for top speed world record, go diesel internals