I'd be tempted to try the 4 into 1 headers, I know that design is quite critical on these and that Volvo R sport did recomend the cast iron manifold but when you look at its design, specifically the way no 2 and 3 ports are only separated by 15-20 mm I'm a bit dubious about it. I've heard it described as a rev limiting design and its possible for one cylinder to suck in exhaust gases from its neighbour. But I don't really know and I suppose you should try both.