I have found, using a few different shelters, and both sil/sil and sil/PU....and also stuff with and without caternary cut, that sil stuff does stretch a lot more than good old PU. Especially sil/sil. It's mildly annoying, but a lot of it is remedied by retensioning the shelter an hour or two after pitching. Not always an option, but when it is, I've been able to get the sag out and not have it come back much.
Caternary versus non, not much difference in stretch, surprisingly.