While I see many people posting how "normal" home port jobs on
OE iron heads can easily flow 260-270-280+, the fact is, those
cases are few and far between. The typical big valve 906/915 type
BB head that was home ported or not done by a shop that is well versed in
preparing racing heads usually flows in the 240's, or less. It's takes
a fairly skilled person with a good understanding of those heads to
get them to flow in the 270's +.
This is one of the main reasons heads like the RPM's and Stealths
so often outperform ported OE heads without any additional work
done to them. The home ported "260+ cfm" heads aren't usually as good
as the owners think they are. Unless they have been on a flow bench,
you'd really just be guessing.

Here are some numbers from some recent heads that came through
my shop for freshening/updating. Neither of these sets of heads were
home ported, but done by a regular machine shop that wasn't necessarily
specializing in head port work. They had the usual big valve install
and some amount of associated blending along with some blending
of the pushrod pinch area.

915 head, older MP 2.14 intake valve with back cut/ new Ferrea
1.81 exhaust valve with back cut, and I freshened the valve job.

Lift------in/ex
.100----70/48
.200---145/99
.300---196/133
.400---227/161
.500---236/181
.600---234/191

906 head, SI stainless 2.14/1.81, no back cut, used valve job
(But not many miles on it) with radius as bottom angle on
both intake and exhaust.

Lift------in/ex
.100----69/48
.200---124/94
.300---180/129
.400---227/158
.500---241/178
.600---245/193

So, it's pretty easy to see how even an OOTB Stealth head
should be able to provide a nice improvement in performance
over these two sets of reworked OE heads, and these are
really fairly typical of what I see.
I will add this, for the average home porter, the 346/902/452 heads
are much easier to get to flow 250+ cfm, and are also a better bang
for your porting $$$ if you're paying to have the heads reworked.