I'd avoid using InteropBitmap for the time being and with this kind of task. I don't think the performance increase would be all you're hoping it to be, especially with the memory leak that requires you constantly calling GC.Collect(). If you said you were doing Direct2D instead of GDI+, I would see more of a reason to attempt going this route.
Is the GDI+ 2D drawing you need to do something that can't be ported over to DrawingContext's methods? In my experience, this has been the easiest and fastest route to porting GDI+ stuf to WPF.