real apps dont scale better because you are getting towards the limit of the throughput used for the cpu gpu ram etc...and that is also not always entirely true for hardware raid. on-board solutions doubly so, yes of course, the limitation of the bus. NOW pci-e is a faster demon. still strangled you say at x4??? well there is a solution for that. a x8 pci-e 2.0 specification raid card. guess what the only one in the world is? pci-e 2.0 x8 raid card?
lsi 9260