I am thinking about upgrading from ported 915 to 440-1C. I have to use iron heads for class rules. What kind of power potential can you get from a well ported set?

Good heads just heavy. I have had mine for 8 years on 3 motors a 440, 400 stroked to 470 , and now a 440 stroked to 500. Gasket bowl ported and a little bowl work. First time I bolted them on ,the car went from a 6.27 to a 6.01. I had my first valve job last year on them , and this year i decked the heads by 0.075 to get a 68 cc and bring the compression up on a flat top motor. Currently running on a unported indy intake in a 2600 lb arrow and running 5.64 et . Good heads for the money, just dont deal with INDY , good venders here on the website to purchase from.

Porting on 440-1 aluminum with oversized valves has gotten me up to 400 cfm flow at max lift. I am not sure if you can get the same results out of the iron heads, but if the castings have the same thickness in the ports, you should be able to.
Glen Knolton of Thunderhead racing in Bethal MN did the valve install, after originally being ported by RJ Performance in Iowa.
Intake valves are 2.30 and exhaust are 1.78 to make room for the intakes' bigger diameter. This setup could yield over 900 hp. Main thing is to have a big enough cylinder bore to make use of what they will flow. If you have the option, go 4.500 bore.
If you are limited to a 4.375 bore, you might look into notching the top of the cylinders for flow, and might be better off with the 2.25 valve, which pushed my dart to the 8.77 in my sig.

800-850hp on a nice effort engine.

8.10 @ 169 2475 NA and 2.25 intake never dyno'd

440-1 alum

hey ts3303 you run 8.10 w/ indy steel heads ? That is flying with steel heads. How big is your motor , a little specs on you motor , i am still trying to find the right combo for my 500 ci.