Should I Swap the Diff?

I've been considering swapping my diff with one with better gearing for a bike-engined car. I have the most common version, the 3,62 ratio. This means that I'm in 6th on the rev limit at 173,5km/h. I do half the Estoril straight on the limit. The advantage is that theoretically I can do 0-100 km/h in 4,5 seconds, doing a good gear shift (never tried this).

In theory, it is possible to calculate the theoretical maximum speed per gear using math. Knowing 1) the engine revs limit, 2) the primary ratio on the engine, 3) each gear's ratio, 4) If the reverse box has a 1:1 ratio or not, 5) the diff ratio and 6) the tire circumference it is possible to calculate the maximum speed on each gear.

Used an excel sheet that does the calculations, if fed with the correct data. The next image shows the difference between using my current diff (left) and the rare/best 3.14 diff (right).

For those that prefer a table with values, here is the comparison in numbers:

Gear	3.62	3.14
Speed in 1st (kph)	73,53	84,77
Speed in 2nd (kph)	101,80	117,36
Speed in 3rd (kph)	128,95	148,66
Speed in 4th (kph)	145,43	167,66
Speed in 5th (kph)	162,88	187,78
Speed in 6th (kph)	173,43	199,94

Well... The trade-off between the two diffs is that I gain top speed at the cost of acceleration. Also, if I get hold of a 3.14 I will get LSD with it. The top speed increases by 25km/h...
If I can get the car on the road, it is important to note that doing the maximum allowed limit on the highway (120km/h) I'll be at 7500rpm with the 3.62 and at 6500rpm with the 3.14. The difference in noise and consumption may be noticeable...

According to this page on Super7thHeaven, I'm looking for a diff out of a Ford Granada 2.0. There weren't many of these sold in Portugal... And I can't even find one on eBay currently, to have an idea of how much it would cost in the UK.

Well... I'm not completely decided on this, anyway...

EDIT 2017-05-27 - Another good reason to change the diff is to reduce the need to shift gears up. For instance, when I exit the parabolic on Estoril, I'm on 3rd (probably) and I'm constantly up-shifting until I'm in 6th because the max speed in each gear in between is just 15km/h more than the previous gear. And the car gets that extra speed quite fast!
The diff I'm looking for is a 7,5" diff (2 bolts on the top and not a single long bolt), with bolt-on shafts, 3.14 ratio. Having LSD is a plus. A similar diff but 7" and push-in shafts (no LSD) would cost around 300€, from an ebay listing, shipped from Latvia... Unfortunately, the inwards of a 7" are not interchangeable into a 7,5" casing.