One thing is for sure, twin scroll is more easily justifiable with a 4 cyl engines. Evo engines for example, with lower power output, lets say sub 500whp, twin scroll will show little to no benefit. But you also have to understand these engines run very high drive pressures in comparison to ours. If you were going big boy turbo in this application, and want nothing more than straight line performance, single scroll is also the way to go. Twin scroll benefits largely in a street application that also wants big power, say 600+ minimum. The big borg turbo's really shine here. Its awesome to be able to say you have an 800hp 4 cyl and a very usable power band. Twin scroll also works great for people who need lower drive pressure but still want to make good power and usable powerband. AKA pump gas power.
As far as journal vs ball bearing, I would always choose BB if I could, but thats not always needed, and of course most times Im not willing to spend the money. Ball bearing works awesome if you need fast response with a larger turbo, such as a low pressure street turbo. It also helps a lot with transient response. But of course we ask how much? Well, if youre in the sub 500whp level, its not as noticeable. When you are using larger turbos for the setup, it only helps. For me, I will always use journal bearing twin scroll turbos in the sub 600whp range, And only will I use BB if the application NEEDS it. Lets say a street strip car that doesn't want to swap turbos between events. They say BB is better at high boost levels... well look at borgs. You don't need BB to run high boost, but it is a band aid most definitely.
