Just redid the plumbing in my house, I have a well so I ran 1 inch copper from the well pressure tank to a manabloc system. From there I fed 3/4 inch Pex to the oil boiler domestic coil (it was originally 1/2 inch copper) and 3/4 inch Pex back to the manabloc. Feeds off the manabloc are 1/2 inch.
Before I was able to take a 30 minute shower without running out of hot water. Now the hot water runs out after only a few minutes. Becasue the domestic coils are heat on demand the only thing I can think of is that the 3/4 inch tubing is passing more water through then it can heat up reliably.
You need some type of flow control device on the outlet I think. On demand hot water heaters if you read up about them have limeted flow rates of ~ 5-8 GPM due to the same issue I believe you're talking about.