great, thanks for all the advice guys; you all hit the bullseye of what i was asking.
i mentioned the word mass, because i knew i'd get the right train of thought going. i knew mass increases the temp rise time; analogous to a capacitor & voltage. from the answers i got & after thinking about it, the normal operating "spikes" i was thinking about are left up to the copper to "filter" out (like a PSU filter cap). i wasn't clear if the 30 to 40min heat up times assumed inital conditions of "normal idle" (no sleeping apps) or cold boot, but even the worst case 30min from cold boot is plenty (considering even the high temps would be not far from ambient).
as for the spike created by a failed pump, the change in thermal conductivity would preclude calling it a "heat spike" as opposed to "sub-system failure". this brought me to another question about redundant pumps:
I am starting with an ehiem 1200 pump, which i feel already has overkill power. if i wanted redundancy:
?will 2 series ehiem 1200 pumps create an obscene amount of unwanted water & case heating?
BTW,i guess i'm not alone when in my attempts to use better definitions to help get a point across more clearly. an air-trap is the more appropriate term (putting pumps in a box to make it a sump is a fringe benefit). i might need to reconsider my airtrap design, since at higher flow rates a vortex might suck air-bubbles back in the lines (like a bathtub drain sometimes does):
flowing R->L a vortex could form at the exit (left side). flowing the other direction might impede the flow somewhat. any ideas?