I> need to achieve a throughput of at least 2 MBit/s, according our
> requirements. Due to the HW this should be possible.
I am not sure why you decided to go in such extreme direction with your changes.
We are almost able to saturate 100MBit connection (>8 MB/s) and upload about 2MB/s from SD Card on STM32F407 with RMII attached PHY (Some Micrels KSZ...)
Are you using some WiFi in your setup? With Ethernet networks we only needed to tune memory in lwipopts, and there was no need to change types and/or polling interval.
Have you benchmarked if the need for optimization really is within LwIP related code?