If you have 3/4 inch line feeding a 1/2 filter it may or may not make any difference. Just because the inlet dia is larger it does not neccessersarly mean the flow is greater. The internal orfices in the device will determine the true flow. The litature for the filter should tell what the cfm flow rate is.
That being said, IF you are connecting a 3/8 air hose to the down stream side of the filter/reg than the 1/2 or 3/4 is of no importance as your flow is controlled by the smallest flow rate in the stream.
Verify the flow rate of the filter/reg you are planning on using, make sure it is adequate for the flow requirements of your equipment with at least a 25% min fudge factor.(use 20 if you should only need 16 for example).
As mentioned, the plastic line will not provide much cooling to your air, you might need to address a moisture problem.
Anytime you can SLOW down the flow rate, the air will set in the container longer and btu's will be pulled out of the stream.
I have no experience with the rapidair product, but rushing air through plastic may cause a static buildup. What type of static grounding is provided, or recommended?
For the short answer, if your 1/2" filter will flow more air cfm then you need, it will work just fine
