I know its a long shot, but could the vmxnet driver be added to RouterOS? I guess the performance and CPU usage are much better when using the enhanced driver.
The one that is currently working in VMWare but not really optimized:
0 device="00:10.0" name="79c970 [PCnet32 LANCE] (rev: 16)"
vendor="Advanced Micro Devices [AMD]" category="Ethernet controller"
vendor-id="0x1022" device-id="0x2000" irq=9 io=0x1080-0x10FF
vmxnet driver source is at /usr/lib/vmware-tools/modules/source after installation of vmware tools. . .
Sam