I think that your ink argument is flawed. You mention that the ink costs more on the 8300 when in fact it's just the opposite. A 130ml tank for the 6300 costs about $75 whereas the 330ml tank for the 8300 is $130, 2 and 1/2 times the ink for $55 more. Your argument about more in in the lines to the head, you're kidding right?
I think the price I saw for 8300 ink was $160, but of course it is still much better. But imagine this scenario. You've just replaced 4 or 5 ink carts, and the other 6 or 7 are like 30% to 80% full. Now your print head dies, or some other flaw actually that makes you really consider if you should pump more money into it or let it retire. Maybe there is a new model that just came on the market with much better ink. Now when you consider you have 5 carts that are 80 to 90% full (hence 330ml *5 * 80%=1320ml) and lets say all the others add up to roughly 40% (7 * 330mL * 40% = 924ml) So it looks to me like this printer has 2L of ink in it. Now if you can sell your just opened carts to someone, great, but how much will you get? I don't think I would be buying an unsealed ink cart from anybody. So it looks to me like this printer is dead with 2 L of ink in it. It is just impossible to reach the end of a printer's life with using up most of the ink inside.
I'm not disagreeing that the 8300 is an amazing deal over the 6300 if you have the space for it. But I just think that it is not realistic to say that you will get to use up all the ink from an 8300 that you get over the ink in the carts on the 6300. And yes, from what I hear, the 8300 does take considerably more ink to prime than a 6300.