We haven't produced any scientific results and won't publish any until we can do them properly (hopefully before GA), but we have done some ad-hoc tests, to give you a taste:
Performance very much depends on your hardware. Using a single quad core Xeon server with 16 GiB RAM and SAS hard drives as the messaging server we can currently hit around 50K *persistent* messages per second, each message is around 1100 bytes in size, that's a data persistence rate of around 60MiBytes/sec which is *very* fast.
We hope to push that to around 100MiB/s which is close to saturating the disk's max physical write rate.
For non persistent messages, on a single server I did about 350K small messages per second (empty bodies) and IIRC around 100K 1100 byte messages per sec.
I don't want to be more specific than that right now, since the above results are not very scientific but they should give you an indication of what JBM 2 is capable of.
Why not try it out yourself?