Several of those questions or deliberations you could have answered yourself; primarily the question whether or not OC-ing the iGPU will increase its QuickSync performance. Obviously it will. How much this is I do not know, I doubt it is very much though. QuickSync is relatively fast as it is. I see no reason why it would increase its quality just by OC-ing. In fact, there is not a whole lot you can do to improve on QuickSync's quality. It is what it is and, quite frankly, should not be considered a serious option if quality is what you are aiming for.
The filter is done on CPU level (as far as I know anyhow, can't imagine it being done on GPU level); however, even Lanczos (arguably the best filter) is really speedy so only consider the others if you are really starved for CPU time. My i5-760 + HD 6870 have no trouble dealing with Lanczos. And whereas my GPU obviously is more potent than yours, my CPU is inferior to yours.
Last but not least; how about just trying? The best way to figure things out is by trying them for yourself. If you are embarrassed or shy about putting out poor quality you can always just go with local recordings and experiment all you want without anyone ever knowing just what the f- you were doing or thinking. ;)
The filter is done on CPU level (as far as I know anyhow, can't imagine it being done on GPU level); however, even Lanczos (arguably the best filter) is really speedy so only consider the others if you are really starved for CPU time. My i5-760 + HD 6870 have no trouble dealing with Lanczos. And whereas my GPU obviously is more potent than yours, my CPU is inferior to yours.
Last but not least; how about just trying? The best way to figure things out is by trying them for yourself. If you are embarrassed or shy about putting out poor quality you can always just go with local recordings and experiment all you want without anyone ever knowing just what the f- you were doing or thinking. ;)