If anyone was intrested the AIX tweak to speed up ATE is as followings:

(It did have a small side effect of adding a slight dely on our AM65 screens every now and again, but these are on a serial card on one RS6000 and TELNET to another RS6000, so normal installs should be fine)

at the su prompt:

no -o tcp_nagle_limit='64'

This should sort it out. If it all goes wrong:

no -o tcp_nagle_limit='65536'

To put it back to system default.

If this setting is effective, you need to make it permanent using smit. this is too complicated to type out here, but it is in the "performance" bit.