The maximum data rate depends on the connection interval set by the master. With the nRF8001 you can send an average of one 20 byte data packet per connection event. With a connection interval of 7.5 ms your data rate would be about 21 kbps. If you need a higher data rate you should take a closer look at the nRF51 series. With the nRF8001 you can receive 3-4 packets per connection interval if the connection interval is 12.5 ms or slower. It is also possible to send a maximum of 2 data packets in one connection event provided no data was sent in the previous event.
I copy-paste from another thread. The answer may be useful here:
When talking from the nRF8001 to an iPhone:
With the nRF8001, the throughput is less, since it can send only one packet and receive two-three packets for each connection event. With iOS, this becomes 0.67 kbps notifying and 1.3-2 kbps when read.
