firstly never ever under any circumstances let a cat out with a lampshade collar on, it totally exposes them and leaves them absolutely vunerable to all and nearly every danger out there !!!
secondly, if the wound has well healed, she's been kept in for atleast a few days then if she got out (without the buster collar of course) i am sure she would be fine, bearing in mind there are internal stitches in the muscle layer etc and it heals quicker in there than on the out side but ideally if poss i would personally keep them in till a day or so after the stitches has been removed, that way you know everything is 100%