Readynas 6 Jumbo frames optimization, SMB versions OS4 & OS6

I have a ReadyPRO 6 that I got last month instead of my previous Ultra 6 that got funky when restarting it. (thanks to extended warranty). I had previously the infrant NV's variants since the company started making them.
I also have the Synology 1812+ and 1010+ and an acer home server (for PS3 streaming). I had also the Thecus one. (7700)
I was always puzzled by the not slow transfer speed on Readynas OS4.xx (and 3.xx before) . But sometimes it was going well.
I tried all the possible tweaks existent on the forums and on the net along this years without really getting it.
Sometimes it worked, sometimes it did not.
When I got the new Pro 6 and put the previous Ultra 6 HD's back, everything worked well, but I got fed up with the "vodoo" involved in the transfer speed.
So I wanted to just get my data from the disks and then sell the Pro 6 unit (new unit that still has a transfered one year warranty)
I was going again through the optimizations, but I noticed that if I transfer from my readynas to the Synology, my tweaks do not really have any influence.
Another thing that helped me was the transfer to my MAC - funny enough, the Mac was transfering at a fast speed when the windows was slow (7 64 bit - but the same happened on Vista and before on XP) .
So it made it clear that there is a Windows problem (the SMB version I assumed, because the network card drivers were working either fast or slow on the same driver version - version of the network card driver was not relevant) Or my switch (a cheap *** trandnet - but I had the expensive metal netgear with no luckuntil it just fried spontaneously - didi not even bother to return it for warranty replacement - too hot))
WHAT made the difference was the Jumbo Frame setting in the network card driver. (I was macthing befor the MTU on Windows with the card and the NAS, or multipliers etc... the netsh commands - not really making any difference)
The difference was made when I chose a 3000k as jumbo frame (keeping the 9000 on the NAS and router).
On OS4 that was the sweetspot for the Windows network card driver . If I put it to 9k (as it logically appeared to mathc the value on the Readynas, I would have got 5-6 times less transfer speed ).
If I put no jumbo frames I kind of had the same result.
The other values 1k, 2k 4k or 5k 6k 7k worked, but not that fast. 8 k was again painfully slow.
So I managed to transfer my 9 TB in about 3 days to the Synology. average speed of 37MB/s

Now, because I had nothing to lose I updated the OS to Readynas OS6. I had it first to 6.14 following mdgm post and the factory default (which happend anyway when you change the OS - no need to specifically do it). Then updated to 6.16 [T5 and T6] and factory default again, just to get all the File system modifications to the latest BTRF status
I then made the MTU value in the OS6 to 9000 ( as it was uniquely possible on the Readynas OS4.xx).
Now the transfer rate is 87 MB/s - a vast improvement.

So my conclusion is that the SMB implementation on the Os4.xx is very sensitive to jumbo frames. I read that is an SMB implementation 1.2 and the new one is capable of SMB 3.0 (though windwos 7 is capable only to do SMB 2.0 or 2.1 I am not sure).
I tried forcing the SMB value only to 2.0 on the Readynas Os6 (you can do that with a plugin available in the Apps) in order to make sure that there will be no problem with windows. But after I tried with the SMB 3.0 active and noticed no difference, I conlcuded that the SMB implementation on the OS 6 is smarter and can scale down to a lower version of the protocol.
So after more than 8 byear of lurking and reading and seeing people come and go at infrant, this is my post that maybe will help others in trying to get the voodoo out of the jumbo frame and speed transfers