The most convenient solution is to enable TCP MSS tuning, and to set the default value of MAP-T interface to 'MTU of parent - 20' bytes. We will think of this. Of course you can override this settings in cli, but as default for the vast majority it is enough.
Do you want to have 900 dl / 900 ul in IPv4 as MAP-T? As I understand from your results download speed is good enough, but upload stuck on 250 mbps. Please send us (to official support) self-test file taken in exact moment when you run speedtest for upload (it's better to increase test time to 1 minute for example to easy your task if possible).