For question 1,
Really it depends on what functions you're using. Something that appears simple (one or two lines in the MathScript Node) could have a lot of code behind it that needs to execute. I would definitely tread with caution and benchmark as much as you can.
And another message from the help that might be of interest and likely isn't too surprising:
http://zone.ni.com/reference/en-XX/help/371361F-01/lvtextmath/msfunc_classes/