At this point I must agree that it would be a wiser choice to go ahead and put in the 1/2" tubing because everything else is already 1/2" and it will make upgrading to a 1/2" block easier and faster in the future.
However dont expect 1/2" performace out of that system (even though the difference would probly not even be that noticeable).
Also with what you were explaining with the annalogy of electricy; is it possible to run say 120V @ 1A over a 24AWG wire?
How about 12V @ 10A over a 24AWG wire? Even 1.2V @ 100A over a 24AWG wire?
Not trying to undermine you (would actually love to learn some more about this subject to be honest

)
Here is a good question:
Say we have 12V @ 20A and that is running through say a 10AWG wire. What would happen if we cut that wire and added in a small run of 20AWG wire in between the 10AWG?
Not testing you; I just want to know...........
MY understanding is that when it comes to a liquid system, you can get a flow rate ( volume of water X speed it is traveling at) of "X" out of a system. When you add restrictions, either more bends (lower speed) or smaller channels (lower volume), it reduces your over all flowrate. You can overcome some of the restrictions one one side of the equation by compensation on the other.
In this situation; we are going from a high volume and high speed flow to a smaller volume with about same speed. Well what happens at the other end? THe volume going though the channel cant change but the speed can. Problem is that our pumps DONT operate as pressure pumps but as volume pumps. SO unless we have a pump that can put some extra "umph" into the liquid (in the form of pressure) then the flow on the other side should only have a high speed but a lower volume.
OK!!!! I am done with the typing!
I guess let the "constructive critisism" begin!