Hi there
I'm far far from an expert so don't take this as gospel, but I thought a twin scroll header helped improve low down torque, bringing on boost earlier but at the sacrifice of all out power higher up the rev range (boost runs out earlier).
For drag race only, I'm not sure you'd see the benefit of twin pulse/scroll.
I've used this for reference before :
http://forums.thecarlounge.com/showthread.php?4894145-Holset-user-threadLots of pages to read inc compressor maps. Not all is relevant but some can apply. Helped me researching Hx40 variations (bought but not fitted yet though, still polishing the damn housing haha)
Hope it helps
Darren