Have you factored in the costs of the power-service upgrades you'll likely need to be able to pull this off?
Or maybe you have enough spare capacity (and wiring) to just add a 240V three 40A dedicated breakers for just the hot water heater?
If the bonus room is over the garage you'll have enough vertical space to install a 5-6 foot drainwater heat recovery unit below the shower to heat the cold water feed to the shower, which would cut the load on the the hot water heater in half during showers. It'll run you about a grand for the hardware, but doesn't add much to the installation costs of a shower & drain. (If the heater were closer you could get even more capacity-extension out of it by letting the heat recovery unit feed the cold-feed to the hot water heater as well the cold feed to the shower.) With a 6' x 3" drain you'll get ~40% improvement out of it feeding just the cold water to the shower. With a 4" diameter unit it'll break 50%, maybe hit 60%. It would take heat the ~100-105F water going down the drain and raises the 50F incoming water to ~90-95F, at 1-1.5gpm cold-side flows, dramatically reducing the mixture drawn from the hot water tank. If feeding both the shower and the tank the potable-side would be higher and temps somewhat lower, but the total heat recovered would be more. Only if your guests are the "endless shower" type would you be running out of showering water if you have one of these installed.
Longer and bigger diameter typcially means better performance within a manufacturer's line, and going as big as what reasonably fits is probably worth it if you're tapping off the main HW heater. Natural Resources Canada pays for standarized testing of different models, and maintains a comparative performance list here. Take ~10-12% off the NRCan numbers, if configured to feed shower-only, not the hot water heater. The US distributor for PowerPipe is EFI, and they will sell them onesie-twosie at wholesale if you open an account. (You can open an account over the phone with a credit-card number. They're a very reasonable outfit- they don't mark up shipping with handling fees, etc.)
If you have enough vertical room for even a stubby one downstream of the main shower near the hot water heater it can pay back reasonably if you heat hot water with electricity at national-average rates, provided yours is a showering family, not tub-bathers. If you heat hot water with propane or oil the payback is even better. But the payback in extended showering time is immediate. (If you lived 300 miles further north you could get a subsidy for installing one.) It pretty much doubles your showering capacity, even though it does nothing at all for tub-fillling.