Doubt it will really matter with gravity systems or even with a pump. You will see get plenty of flow. Like I said, if you want full 1/2 ID, look for a 5/8 compression fitting and get some 1/2 copper from Lowes or HD. Rigid copper is 1/2 ID, the soft stuff is 1/2 OD.
You could try a copper 1/2 npt to 1/2 solder fitting and get a copper elbow and a copper 1/2 ID pipe for HD. You will screw the 1/2 NPT fitting into your weldless bulkhead. Then just slip the elbow with the tube on it. It might make a good enough seal for you. Test with water first. For under 10$, it might be worth a try. You could solder the pieces together, you will just have to screw the weldless fitting with the dip tube all together at the same time, not a big deal actually.
If you never sweated copper before, this is a good project to learn on it. I learned to do it making brew hardware and save my self a ton of money fixing my house recently when I had to replace a hose, sink, etc.
HomeDepot fittings( these may not be the exact ones, some have shoulders, but give you an idea)
Screw this into weldless fittings, if you have a thread end in the kettle, use the second option.
opt 1:
http://www.homedepot.com/h_d1/N-5yc...copper 1/2&storeId=10051&superSkuId=202904880
opt 2:
http://www.homedepot.com/h_d1/N-5yc...copper 1/2&storeId=10051&superSkuId=202905001
Solder or slip this onto the fitting above(you will need a small piece of 1/2 copper pipe or get a street elbow that is designed to fit inside the above fitting)
http://www.homedepot.com/h_d1/N-5yc...copper 1/2&storeId=10051&superSkuId=202904973
1/2 copper pipe into the elbow above going straight down
Leave some gap from the kettle bottle to the fitting, maybe 1/4 to 1/2 inch.