The problem here is that there are no hard statistics on women's performance in combat because... women are rarely sent into combat. If they are, it is ad hoc and usually the side doesn't admit it, which lends itself poorly to studies.
The only time I have heard arguments about why women shouldn't be in combat backed by statistics was in a study (which did not actually comment on whether this was right or not) which mentioned some information from the IDF. The IDF found that:
1.) When female soldiers were wounded or killed in combat, a significant number of men would both become less likely to obey orders, harder to control, more aggressive, and more likely to attempt to kill the enemy.
2.) Members of groups such as Hamas etc etc were significantly less likely to surrender to a woman and less intimidated by women.
The source is David Grossman's On Killing.
That being said, those are more an indicator of attitudes and sexism rather than of anything inherent about women, in my opinion. Regardless, this remains the only study I have ever heard of (and I looked into it quite a bit a while ago) that justifies women being excluded from combat positions. I couldn't find any studies in support of it. So, personally, I'm inclined to think they're not really out there...
I'm for allowing women into combat positions, by the way.