I've never found the need for a high-powered RIMS element, unless you are doing on-demand sparge water heating. My RIMS heater is 2,000W, which only pulls 16.7 amps at 120V - a 20 amp circuit will handle that, which are pretty common in kitchens and/or garages. Maintaining rest temperature takes almost nothing to do. At full flow/full power when raising temperature from 152°F to 167°F, I see about 6°F difference between what's coming into the RIMS tube vs. what's going out.
There are a lot of other factors that will affect the heat-up rate for your mash: the flow rate you are pushing through the RIMS tube, length of the RIMS tube/heating element,the size/shape of your false bottom, the size of your mash, types of grain, piping diameter, etc. The typical Chugger/March/Riptide pumps we use can't move liquid fast enough to significantly increase your flow rate with the typical 1/2" tubing most people use. Given all those other system limitations, it's tough to get the full capability of a higher wattage RIMS heater for step mashing. Not saying it can't be done, but given how difficult it is, and the fact that most malts are highly modified and don't really require a protein rest, I think it's unnecessary.
For the boil kettle, heater power really only affects how long it takes to bring the batch up to boiling temperatures - maintaining a boil can easily be done on 2,000W or less. My kettle heater is 4,500W - for the boil, I dial the power down to 30%, which is around 1,350W. Insulating the kettle and/or keeping the lid on during the heat-up will keep more heat in the wort and speed up the time it takes to get up to a boil if you really want to improve your heating times.
Like most things in engineering, there are trade-offs with how you set up your system. It all comes down to what is most important to you: heat up rate vs. staying with 120V. You can't do both without some significant power increases, but you can brew some pretty good beer regardless.
