that is a good point. however much power you can put into the RIMs tube, your pump needs to be able to circulate the water fast enough so you dont overshoot your max temperature (170*).
the maximum flow rate of your system should be a known variable, so then all you need to do is calculate how many watts will heat the water by how many degrees in one pass. lets use some easy numbers like
10 gallons of mash, 2000 watt heater, and 1gallon per minute flow rate...
2000 watts is going to raise 10 gallons of water by around 82 degrees in one hour, or 1.36667 degrees per minute (total). at 60 GPHour flow rate, its going to circulate the entire volume of water 6 times over the hour, or once every 10 minutes. so in 10 minutes, were assuming (for simplicity) that all of the water will make one pass thru the RIMS tube. in 10 minutes (one pass) there should be a 13.6 degree temperature change.
so you can reasonably expect that the maximum difference between input and output temperature (one pass) of the RIMS will be around 13 to 14 degrees on a 2kW element, at 100% power and 1 gallon per minute flow rate.
now how do you figure out how much power is the most you can use?
ignoring the limits of element size and reasonable wiring requirements- to figure out the maximum element wattage you can use, you find the maximum temperature change (170* minus 104* starting temp = 66*). so the maximum you would want to heat your water in one pass at any time would be 66 degrees, or else you would be overshooting, or never be able to run at 100% power (meaning your elements are overkill).
66*-change in one pass(10min) = 396*-change/hour
10gal x 8.3lb/gal x 396 = BTUs required for the change in temperature
=32,868 BTUs
=9626 watts
*i know its not possible to get water to 396 degrees, the formulas dont consider boiling points of specific liquids, or other non-linear things like that. we are only calculating rate of change, not specific temperatures
so at 1GPM, to heat the water by 66 degrees in one pass thru the RIMS you would need about 32,868 BTUs/hr, or 9626 watts of power. you can use simple math to scale any of the variables up or down.