It is worth getting or making a QCTP, not having to mess about with shims. No, 09's are about 10x more durable than 06's. I use both and have a good mix of decent brand inserts, just I dont yet have a right hand external holder for them.
"So how is a bigger bar going to help if this is the case?" Here's some photos. The 8mm one. The 12mm one You should just be able to see that the cutting edge on the 8mm tool is comparatively lower from the top of the tool than the 12mm one is. I'm not messing about machining down the top of it as its not the right size anyway and there wouldn't be much of it left! The bigger bar size is the right size for the lathe, forget about the 8mm one, it's an odd one that came with it. Here's a photo of my tool post @northwest
Yeah but, no but, yeah but, no. One of those is brazed tip, the other uses an insert. What makes you think a larger bar of the same style will have its tip in a different place relative to its centreline than a smaller bar? In extremis, a 50mm dia. bar with the tip 10mm from the top would have the body of the bar bulging out past the tip so would never cut. Have a look at: https://www.sandvik.coromant.com/en-gb/products/Pages/productdetails.aspx?c=S12M-SCLCR 06 . The tip coincides with the centreline of the bar. You can change the S12M in the part number to 8, 10, 16, 20, 25, 32 but the proportions remain the same. Another option is to make this: https://smithy.com/accessories/NDctNTIw which puts the top of the bar above the bottom of the toolpost clamp.
^^^^^^^ as above ^^^^^^^^ You chose the size of the bar to be the largest you can get in the hole you are boring out, and obviously that will fit in the tool post. Go for the largest possible bar and reduce overhang as much as is possible, thin bars and long unsupported lengths make for flex and poor finish which is obvious but still needs to be thought about. If the tip was at the top of the bar it would reduce the size of bar able to pass through a given bore and place the tip at a very odd cutting angle.
