The material alone doesn't improve spool, its all in the design of the wheel which helps, that said, if the dynamics of the billet and cast wheel were indentical, the billet would not spool better than the cast. Billet has a higher boost capability and thats about it - even Borgwarner say this.
Not sure about the RT twin scroll kit as it doesnt look like many people run them, so there isnt many dyno plots.
We'll be posting the results of our new kit soon, which will give TD04 spool and GT2860 power.