Well you have the TCP (tn3270) version that WireShark is able to understand, so I am not sure that is entirely true.
I can see what you mean with regards to the SNA.
Yes - we can "understand" it in the same way as Wireshark. But that is on a packet level.
You can't really tell what goes on in the window. At best you could dechiper/decode the OIA codes (I think we had that back in Network Vantage) but again, that has nothing to do with transactions or response time.
Example: You haev a 3270 screen and do some stuff there. You press "Send/Enter" and you have a brief delay Before you get some feedback, either in the screen or in the OIA. That will not correspond to a application response time - more like the response time of the Gateway or the FEP (if they still use them).