The water is going to travel the 'easiest' path no matter what.

Now I'm certainly not an expert in fluid mechanics, but I believe overly small outlets would tend to negate inconsistencies in the way your block will flow. We could think of this in electrical terms, since that will be easier for me (:)), and they both follow the path of least resistance in a circuit. We have a parallel setup, with two (hopefully) small value 'resistors' being the two easiest paths between your inlet, and each of the outlets. Now also in parallel we have two more resistors, your output barbs. Note that they are in series with the previously mentioned 'resistors'.

Now, let's say the two paths through your block have a 'resistance' of 100 ohms, and 120 ohms, respectively (we're assuming the block pretty heavily favors one path over the other, but your block actually looks pretty darned good, and this shouldn't be the case...). We'll also toss in a value of '12v' for the 'potential' of your water, to make the math easier.

The circuit will see a resistance of ~54.5 ohms, at the block, assuming no other restriction, because of the use of very oversized barbs; this could be compared to the '1/2" everywhere' case. With these values, total flow is .22 amps, with the 100ohm side of the block getting 0.12 amps of current, while the 120 ohm side would only see 0.1 amps, a 20% difference!

Now if we assume the outlet barbs introduce their own, very large resistance of 50 ohms each (let's call them 1/4" OD), we have the parallel paths with 150 and 170 ohms resistance, each. The whole block then introduces 79.7 ohms of resistance into the circuit, which gives an overall flow of a measly 0.15A, with the sides getting 0.7A and 0.8A--a 14.3% difference.

So...long story short:
You could help balance the flow by using smaller output barbs, but it doesn't help much (and greatly restricts overall flow).

On the other hand, using oversize barbs would magnify an imbalance in the flow of your block, but not by much, and it certainly can't hurt overall flow. This would also allow you to run the same type of hose everywhere, to simplify things.

Got the block hooled up with all 1/4"ID barbs and the water flows very well. Flows fairly even through the two outlets aswell.

I found another "Lemon" block already on the market:

http://www.dorrellco.com/lemonb.jpg

Ah but that one doesn't have that square post over the core, like yours. You'll get better results, than that cheap imitation!;)