it depends what type/grade of stainless Jonny, there are various grades and they will all have diferent weights
T304, sorry, should have mentioned that..
I have found out the following..
Steel 8.0 grms / ccm
Ti 4.95
Kind of helps, as the Ti exhaust would be only 1mm thick it should be a lot lighter! Take in to account the relative weight difference and that should make a hell of a difference % wise.. Just need to find out how much the centre section of the current exhaust I have weights up at..