Yeah if you do the maths the 5/8" inlet and 4 festo outlets add up to the same internal diameter on splitter-x, and there is also the increased wall surface that will make a slight increase in flow restriction.
The reason for splitting is to aid overall system flow. If you piggy back three blocks in series then you will probably create greater overall flow restriction, I don't think it matters if the splitter is combined in the block or after. If you have a sound design then a splitter CPU block can work fine, without running it through more barbs of a separate splitter unit.
Its hard to state anything as a fact because what will work well with system X and Block set X may not work as well with system Y and block set Y, given flow rate Z.... there are just so many specific real world variables. If I had Two blocks only, CPU and VGA, and the VGA was definitely not a flow restrictive design, including the plumbing to and from it, then in series would be best with CPU block first. If it were likely to be flow restrictive I'd favour the extra NB block as well and splitting in or after the CPU block as said, that way the coolant after the CPU block has two paths that should add up to a greater overall flow diameter combined so aiding overall system flow.
Of course it may also make no actual measurable performance difference at all.....
If the two blocks after the cpu were poles apart in flow restriction, (and I can't see why they would need to be), then yes it could possibly affect the performance of the CPU block, but a set-up like that is likely to be flow restricting anyway. Like I said considering the heat added to the coolant in one pass is under 1C, assuming sound design and decent flow rates, it would have to be pretty severe, or combined with weak flow rates, to make a difference. Assuming similar block flow designs and tube runs a VGA & Nb block should be very similar. My system has imbalanced flow on the Splitter-X outlets but as the flow rate is very good and the coolant entry to the CPU strikes the base at the centre it shouldn't affect performance at all.