Thanks Roger, we updated your suggestion to our network team, what we found out they did changes for MTU at Cisco router level and it didn't worked either. The one thing we found out after careful analysis of ethereal trace, that difference of SSL record length at BigIP (16k) and POS (9k) is not a problem at all, as the record length never hit 9k at any time. Any way we are implementing an alternative solution kind of dirty work to download small chunks many times, which is a performance issue, but can be operational. Appreciate if you or anybody have any other suggestions.
Leaving this problem unsolved, kind of sad and unchallenging....but given time frame and customer satisfaction we have to live with that.