f5 BIG-IP working as IPS

AFM has an IPS now, Protocol Inspection. It provides protocol compliance checks that implement a positive security model (the traffic must match or it is alerted/dropped/rejected), and signatures that implement a negative security model (matching traffic generates alerts/is dropped or rejected). The signatures implement a subset of the Snort rules language syntax, but the matching engine is different. There's a subscription service available for updated signatures, and users can write their own custom signatures. Custom signatures are a pain due to some validation bugs, but they show a lot of promise.

As a drop-in replacement for an industry-leading IPS, it's probably not viable at this point. As an enhancement where there's already a BIG-IP, yeah it could completely avoid the need to add another device.