Setting it up as a pre-heater to the tank would cause it to short-cycle on every draw losing efficinecy, even though the hot water tank could have handled the load without a burner firing. But as a post-heater, as long as the output of of the tankless is set several degrees lower than the setpoint of the tank it'll only fire when the output of the the tank drops below what the tankless is set for.
But even this can be problematic. Tankless heat exhchangers are fairly high-head, and you may run out of flow capacity due to the pressure drop before you run out of burner if you're trying to run the whole house through it. A big 'un could handle the 9gpm, but if 3 other taps are open the pressure drop could take it down to a dribble.
A drainwater heat recovery unit (if it can be plumbed in, which isn't always the case) can roughly double the apparent capacity on your showers (and save on the fuel bill), but does nothing for your tub-filling problem. It may make more sense to set up a tankless as a dedicated tub-filler + high-flow shower, where it'll never short-cycle (all draws will be relatively high volume), for maxiumum efficiency, where the quirks of a tankless (cold-water sandwich, etc) are less problematic. A 199KBTU/H may still need a drainwater heat recovery boost to get you the full shower flow. But it could also be set up as a post-heater to the tank on that branch, but using a mixing-valve between the tank and the tankless to deliver ~80F water to inlet of the tankless (so as not to deplete the tank so it can serve the other water-use zones), yet have sufficient burner to reliably deliver full flow to the shower.
It's all kinda klugdy though... If you have a forced hot water heating system with a 250kbtu/hr + sized boiler you might consider installing a indirect fired HW heater instead (or in-addition.)