It works! My productive system now achieves 1732 updates a minute in non-debug mode. Just for the record the configuration I’m using now is:
modbus:poll=5
And like this for 13 different slave IDs:
modbus:serial.someSlave.connection=/dev/ttyS0:38400:8:none:1:rtu:280:3
No errors, just literally thousands of state updates 
I haven’t tried debug mode yet.
Just wanted to let you know as quickly as possible. Great work. Thanks a lot!
Best regards,
Max