I was waiting for one of the pros to respond...but, since you haven't had a hit for awhile, here's my thoughts.
The boiler has a larger output than many standalone tanks, so getting the water in the tank hot enough isn't an issue, it's about how long it takes and the first hour recovery rates. The spec sheet is only valid on the time and flow when you have at least their specified heat input to the tank. Because the boiler's burner is larger than that in a typical standalone tank, you can often go smaller. But, when trying to fill a large tub at max speed and a smaller boiler, you will not get the first hour volumes in the spec sheet, so larger may be required. This all assumes you put the IWH on a priority zone so it can get the full output of the boiler (and typically, it would also fire to what may be a higher temp than when space heating).
I have no experience with that brand IWH, so can't comment on how good or bad it is. I have a big 6' air tub, and ended up with a 50-gallon IWH. I can take essentially an endless shower because of the recovery rate, or fill the tub up without issues. Other than initial costs, a good indirect WH can have very low standby losses - some of the better ones are rated at less than 1/4-degree per hour and have a lifetime warranty against leaking.