I don't think the speed is a deal breaker.I'm quite comfortable with 21 kts. The requirement for high speed intercept will be met by either the 11mtr RHIB or embarked air.
For 95% of their operations these ships will be patrolling at 12-14kts thereby enjoying fuel economies and comfortable sea riding.
We tend to forget that many of the OPV predecessors in the "patrol" function, including the Bathurst Class corvettes had operational speeds ranging from 9kts (Bass/Banks) to 24 kts and all were proven to be effective minor war vessels.
But the larger sea axe form offered, faster speed, lower drag in realistic conditions and therefore better range at a variety of speeds. Damen could have put in a very exciting offer based around the 1800 sea axe. Perhaps cost prevented them from doing that.
Interestingly, the South African Damen 1800 sea axe are expected to make 30kt.
21-24kt is fine IMO for an OPV. I would rather have a OPV with aviation capability and 21kt to speed, than a 30kt ship with no capability of embarked aviation.
Helicopters are proving essential to anti-piracy and enforcement zones such as the Persian gulf. Being able to support a boarding party with some sort of air will be essential (and a UAV can do this). If only for situational awareness reasons, even in non-combat situations. Of course in high threat situations being able to embark a manned helicopter again, changes the capability significantly.
It might be that the ADF and the RAN might consider a small number of corvettes (4-6) to replace some of the other smaller vessels. There would be a number of 25kt+ ships, that would be ideal, still around the 2000t region.
The 12 OPV's are then really free to be OPV's.
In such a context I would just be upgrading the 25mm bushmaster to the new 30mm bushmasters (RAN wide) with the coaxial .50 cal.