Thank You, David and Matteo for your response.
It seems is hard to decide… 
Since we are in Romania and the customer in Russia I would like to avoid any system issues…
This year we have one PLC with about 8000 Tags. Then will follow other 35 which will bring to Ignition more than 30000 Tags, so at the end will be big enough to not want communication system failure.
@DavidWisely: Do you know if the issue solved for you by TANI are already integrated in TANI installer or if I will use TANI I will face the same issues which will be solved by TANI support?
The idea is that I’ve “played” with TANI OPC and I love it since you don’t need anymore to disable "Optimized block access " and you can browse the PLC and easily the symbol with the same name in PLC.
@mfontana : Matteo could you please tell me the order code (MLFB) for the Siemens OPC server you used?
Thank You both again,
Adrian