I was afraid to ruin the fabric, so I made a muslin (yeah, I can't believe it myself: an extra step, and I'm not too lazy to take it). Anyway, the muslin fits nicely, everything ok -- the only modification for fit I decided: a bit more ease at my hips.
So, all confident, I went ahead and cut the silk (and put it on a wool blanket for cutting, which really helped with the slipping!). I decided it needed a bit more stability, so I added interfacing to the outer yoke (does one normally put the interfacing to the inner or the outer or both????, anyone?).
And now, front yoke and front body don't fit together. So I will need to go back and unpick the basted front pleat to make it fit. Note to self: invest into a felt marker pen for fabrics where tailor's chalk disappears as soon as you apply it. or mark with thread or something.