the usb packet size should be 1024 for SS, 512 for HS and others 64 bytes in bulk transfers. You may increase if you use bursts 16 (is max). Have you a specifically reason to try it with the bulk loop auto example? I am not sure if these example is optimezed to check traffic throughput. You may check it with the streamer app and the UsbBulkSourceSink example.
Yes. As Lumpi already told you, use UsbBulkSourceSink and streamer application to measure the throughput of Bulk transfers. And go for UsbIsoSourceSink example if you want to measure throughtput of ISO transfers.
We have seen more than 400MBps with bulk transfers.