The gain might suprise you. One thing you guys are overlooking a bit is the reversion into the other cylinders from the factory log style manifold. Having the exhaust seperate all the way to the turbo should help other cylinders pick up power.
I've got the old JRB header. The tubes are only 1.25" which are a bit on the small side, 1.5" would be perfect I think but a tight fit for the design. In fact The Pope's exhaust manifold flows 1-5 cfm better than it. But OMG, My true S60 spools up at 1700rpms. 500rpms sooner than it used to. I did change heads and the intake too but I still believe seperating the cylinders made a big difference.