I feel like you haven't seen much of Kostjasyn then... He's very very average in attack. His job in the Cowboys team is to be a tackle bot for 30-40 minutes and give Granville a rest so he can come on and tear teams up through the middle.
I feel like Randell has had a good solid season without getting to many wraps for it. He's improved a lot in attack and defence form where he was last year and has out-performed Levi, albeit in less minutes. Our defence fell to shit when he came off last week against the Panthers, that should tell you something. Dummy half was never his preferred position and it was always going to be a slow process to get him to be a competent first grade hooker but I feel he's improving to a competent first grade level and even has the capacity to improve even more. As opposed to Kostjasyn who is never going to get better then what he already is. If Kostjasyn is indeed better, it's not by much and Randell is 6 years younger.
I'd say Brown's decision is based on the fact it would be a salary cap improvement for us. Tyler Randell is on loads of cash and somehow a premiership winning player in his position is worth less (nice job Hages/Stone)... Couple that with the fact Kostjasyn is experienced, comes from a winning culture and actually wants to come to the Knights (silly bloke). So yes, in that regard, it's the correct call to make and I'm happy we're going through with it. If Randell was on 100k like he should be then I'd be totally against this decision.
Just don't expect miracles from Rory. Expect Randell levels of performance, don't get your hopes too high.