With 8x18" wheels running 225/40 tyres, std gearbox, and a revlimit of 7600 rpm your theoretical top speed is 188.78mph running a std ratio.
Same setup running 3.9 ratios is 175.23
Also what would a front diff cost to match? and if i didnt change the front diff would i really cause major problems?
Yes you will damage the centre diff. As i said before, you have 2 options, either replace the CWP in the front diff to 3.9 OR get a step off gear+chain to go in the gearbox, either option means the centre diff will work.