Yes it seems that over time the tack line is slipping a little because I'm sure I pull it untill the stop. However when we do the test we've around 45deg now.

"How did you set the luff tension line inside the spi luff? On a new spi I hoist the spi with a slack tension line, then tension it about 5cm" --> I've no clue at all what you mean????

And yes the difference in age was most certainly there as we use an original spinnaker from a boat from 1997 (however it was not used too much before we bought it) and they were a sponsored race team. (so probably a spinnaker from 2012 or 2013) However the difference was realy large. (see underneath picture as an example) can this make such a difference?