Run the BIX in parrallel.... you will get much better performace. Running blocks in series is alright but definately run your radiators in parrallel. Reason: radiators work the best when the difference between the water temp and ambient are greater. In series, you are cooling the water down and then trying to cool it down some more when it is already cooler; hence the second radiator will only be running at say 25-50% efficency. Running them in Parrallel will deliver maximum difference to both and decrease the time in both radiators at the same time!
Also reason why you want a larger pressure line to the CPU block is because that is where the most water flow is needed (higher heat disapation). The other blocks combined dont even put out 1/4 of the heat that your Pelt + CPU will be putting out.
hehe if you can do a full 1" return line, ! would say Go For IT!
couldnt hurt. But keep your pressure line smaller for sure.