So thinking of buying and remap tuning a L494 SDV8 but Alive are advising that more torque could blowup the gearbox, any thoughts welcome pls ?

The gearbox isn't "weak" as it's capable of handling the standard 700Nm / 516lbft put out by the engine. That is the same as the SVR puts out, after all.

Alive are being honest and upfront - adding torque will stress the gearbox as it's rated to 700Nm (plus whatever safety factor ZF build in).

Presumably you could make a clever map that just boosted the torque to 700Nm in those bits of the rev range not already there. But as it makes 700Nm from 1750 - 3500rpm, there's not much scope to do so.

If you want faster than a SDV8, buy a 5.0SC.  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed