XBee 900HP 200k data rate

Hi, we are currently trying to evaluate the effective data rate for the DigiMesh Xbee 900HP 200k module (S3B). Using transparent mode (AT) in XCTU throughput test, we are able to achieve approximately 89 kbps with a data payload of 10000 bytes, which is close to the documented data rate of 91 kbps (Unicast, no encryption). However moving into API mode, we found that we are having difficulty attaining such data rates, primarily the issue when designing our own throughput test is that we are limited to the 256 bytes payload limit of the message. We realised that in transparent mode, using 256 bytes random data, we can only get 33 kbps.

The question is, how does transparent mode allow data payload size of 10 Kbytes. Is there any differences when running the throughput test in transparent mode as opposed to designing our own throughput test and using API mode?

Any suggestions on how we can do a throughput test using API mode properly are most welcome!

Thank you very much!