I thought I'd add my 2c.
I have had one of the old Batmbl split dump/front pipes on my 33 for a few years and when the turbo was stock I never had any boost issues or anything.......but, once I had my turbo highflowed I have had boost control problems with spiking and the highflow kind of hiccups and has a little stutter high up in the rev range.
My tuner (unigroup) say that this is caused by the split dump being made for the standard turbo so when the rear housing gets machined out a bit for the bigger impeller thingys (very technical speak..haha) the "hole" in the split dump is actually a little smaller than the hole in the exhaust housing which in turn creates enough turbulence to create small boost controll issues.
If someone with a better understanding of turbo flow etc can explain this in a better way please do.
I hpe this makes sense, I think you'll know what I mean though.