Hi Julie!
I don't have experience with the card but ther are some (small) downsides.
You have to buy an interface card for each UNIX client.
Each client requires a port on your Escon Director.
I don't think TCP/IP for MVS can deliver you that kind of throughput. We have a 
Cisco router connected to our OS/390 server and I cannot get it faster then 
about 2 MBytes/sec.
IBM indicated that one cannot get much higher with TCP/IP 3.2.
Kindest regards,
Eric van Loon