I was just curious to know which variety of decat downpipe flange Is best, single rounded neck (which blocks westgate) or open neck. I've been told that the single rounded tends to help prevent turbo spikes, but might hamper your performance if you want a big power setup. There's also a twin dump downpipe, are there any benefits from these?

 

I would go for either twin dump or open neck.

I have an open neck d/p with a splitter mod (piece of metal welded in to seperate turbo/wastegate gasses) - Hayward and Scott made mine - can't complain at all!