Shinji failed to kill an injured, one handed, unreleased Grimmjow with his strongest Cero.
What makes you think his strength is enough to dent Ulauiorra?
From what I gathered from that battle, it appeared as though Shinji was merely "testing the waters" w/ Grimmjow, a strategy that has been implemented by the shinigami Captain classes time and again in most of their major battles. Basically, he wasn't taking Grimm seriously and based on how nonchalant he was during their skirmish, I don't believe he was fighting at full power either.
Untill we see Shinji's Bankai we will know.
Also I think Vizard Shinaki Shinji will give him a good fight.
Oh wait I just realized my error.. That reversal trick isn't Shinji's bankai, it's his shikai right?
Yup! That was his Shikai.
@Carter_esque: Shinji himself says that he wasn't holding back when he used that Cero.
Ulquiorra in R1 alone could tank an FP Getsuga Tensho from a much stronger masked Ichigo without getting a scratch. R2 should be able to tank Shinji's physical attacks without any probalem
As I wrote before, it would depend on how quickly he adapts to Shinji's shikai. Don't forget that Shinji also has a number of effective kido spells at his disposal if his physical attacks fail. He could utilize any of them while Ulquiorra's trying to tell left from right.
Shinji loses if he does not release, even if he has his mask. He failed to put down a one-handed base Grimmjow who had no idea Shinji's power would spike so much and was grievously wounded by Vizard Bankai Ichigo. And his TBTP feats aren't good enough to make up for his base power, maybe he can hold his own against Ulquiorra for ten seconds straight before getting beaten. Eventually, he's going to get a sword through his neck.
Shikai Shinji wins.