auto baud issue with ADBF706WCCPZ4xx

I am trying to complete the auto baud process with a ADBF706WCCPZ4xx before using it in slave boot mode. At 128800bps a bit width is ~7.76us, the device is responding with a ~7.67us bit width which works well. If I go up to 250000bps a bit width is 4us but the device responds with a 3.4us bit width and thats not going to work. At 1Mbps the device is responding with 858ns bit width or 858kbps. Whats the limit here? I've got a 500k image to load at boot and I'd like to go a little faster than 128k.