Over what distance is that 1 degree calculated?
I have seen many different calculations showing that it is within measurement tolerance of no difference
As for tube adding more restriction, that is something that should be taken into consideration when sizing the pumping power required. This is why it is suggested to minimize the length, not only does it look better but also reduces any unneeded restriction.
If they go twin D5's there is going to be no issue with overcoming the slight restriction you mention. For that manner I do believe that slight temp difference between 1/2" and 3/8" is negated when flow is increased, which twin D5's will do happily.
In my sig is a guide to sizing the pumping power required and HERE