The modern boilers with outside reset often do not run at max temp. Your old one might have. Being oversized, and at a higher temp meant it might keep up with the demand on the IWH. The Ultra, on the other hand, if set up reasonably, probably will never run at max temp unless it just can't keep up at those lower temps. This is likely to mean the IWH may not get as much heat as the old setup. Most of them are designed to provide priority zone for the DHW, which does two things: runs the boiler up to max, and shuts off the other zones until the DHW is restored. Some have a max limit, so it will switch back to space heating if the call for DHW is extensive, but not all. In general, that max override is not needed. While it may work without the priority zone, if the logic is calling for say 130-degree water for space heating, and the tank wanted to be 140, it would run forever and not achieve the setpoint for the IWH - the tank can't get hotter than the supply, and setting it up as a priority zone forces the boiler to high fire to get it there as fast as it can.