OK... Actually you can barely see any performance difference between 1/2"ID and 3/8"ID tubing over 1/2"OD barbed fittings.
That's because the inside diameter of 1/2"OD barbed fittings is 3/8"ID.
So in other words the restriction is mainly caused by the inside diameter of the fittings itself...
The advantage of using 1/2"ID tubing is less back pressure on the pump due to more volume.
The advantage of 3/8"ID tubing over 1/2"OD barbed fittings is neater looks
So if you want to use 3/8"ID tubing over 1/2"OD barbs, i highly recommend you to go with 3/8"ID x 5/8"OD x 1/8"Wall tubing... 'cause 1/16" Wall tubing kinks pretty easily...
oh and also you may need to heat up the ends of 3/8"ID tubing a bit, so that you can easily insert them over larger 1/2"OD barbs... using a hair dryer @ low setting or inserting the ends in to hot boiled water will do the trick
Bruce