Unless you have a lot of 90s in the black iron it shouldn't be much worse than 1" pex. It is usually over 3/4" inside and 1" pex is only 7/8"
It sounds like it is just the large volume of cold water that is the problem. Is there any way you can modify the return pipes from the radiators? If you can do that, putting a zone valve in it with a small bypass around the zone valve would keep it from dumping all the cold water in so fast. A strap on aquastat could be used to open the zone valve once the returns are hot.
Another option that would probably be kind of complicated would be to get some type of a pulse controller so the pumps just run for a few seconds on and off till the cold water is all flushed into the system.
Probably the easiest is if you have an unused supply port on the boiler, run a pipe from it to the existing return port and put a tee onto it for the existing return and another pump (probably taco 007). Then put a strap on aquastat on the return pipe that turns on the new pump and have it come on whenever the return water is under 150 or 160. If you use large enough pipe you can easily get 2-3 times the GPM that is returning from the house. So if you are getting 6gpm from the house at 110° and 12 gpm from the new loop at 180° you will have return into the boiler about 155° to 160°.