Little bit off-topic for this forum ... support for BIG-IP 540 has been added in version 9.2 which was released in September this year. So, the your previous information was correct. See version 9.2 release notes for details:
Click here.
Regarding rules, direct node select expressions and performance... Wow, you've must done very detailed measurements that you know for sure that you can add 2 if-statements (and no more) before your performance is impacted. You either must have very complicated expressions in your rules or extremely strict performance requirements that adding 2 if-statements results in un-acceptable performance degradation. BTW, what are the performance metrics we are talking about? Throughput, connection rate, transaction rate, number of simultaneous connections, latency, something else?
I'd like to reiterate: implementing the same logic using direct node select expression (which is most likely not possible in you case anyway - can't tell for sure because I've not seen your rules) would have the same performance impact as using a cascaded if-statement. Direct node select expressions use the same evaluation engine as iRules.
I've outlined in my previous post a possible solution to your problem Please, explain why it cannot work for you.