No one knows exactly, to my knowledge, and it is part of a whole system so that makes it tough - a LPH rating is still just a generalization. A lot of tuners seem to say it is like a "stage one" setup ... in that it flows more than the other N54 cars with a stock setup. If you are FBO on just pump gas, and all your stuff is healthy you might be fine. Turbos or ethanol mix, forget it.
These cars are old enough, anyway, that changing the pump is just not a bad idea. The lack of a LPFP sensor complicates knowing if there is a lift pump capacity issue when tuning. Another reason to go proactive. The downside is the E89 is underside mount for the pump, which sucks. One can order an LPFP upgraded bucket from several vendors, or you can roll your own. IMHO easiest way to roll one's own is to just buy a used bucket on eBay and build it up.