i was just wondering weather it makes a difference to performace if the diff and gear box dont match up? does it make any difference

The gearbox and diff are chosen for the selected engines torque characteristics. There is a bit of an art to it, but I've never heard anyone not be able to change their driving styles to suit.

Putting a 2.9:1 diff on a 4 cylinder will almost certainly make for very poor 1st gear driving.

I run a 3.45:1 diff in my 1200 with a ca18det, and I can use first gear to go around 90deg corners, and have the torque to get the car moving from standstill.

But for a standard a12, it's lack of low rev torque means you are more likely to want a 4.1:1 diff.