Coming from someone who has ported their fair share of 289 heads, those look really good! I ported mine without the aid of a flow bench, and they did pretty well. They pushed my 3300# pump gas, flat tappet 6800 rpm 306ci mustang to a best of 7.48 @ 93 in the 1/8 naturally aspirated, and 6.74 @ 103 with a 200hp plate. Toploader 4-speed car, and served as a daily driver. Car ran 10.63 @ 126 in the 1/4 on the bottle.
The new 306 built with better heads is pretty much the same as the old engine, except for the heads. (Twisted Wedge 170's I ported... don't have flow numbers, though) It didn't pick up much n/a, but went 10.15 @ 134 on the bottle. I guess the little 289 ports just didn't have enough cross section to flow all the nitrous on top of what the engine was sucking in on it's own.
But, DID seem to do well enough to handle a n/a 306ci engine shifted at 6800 rpm just fine.
I had the heads flowed after a decade or more of abuse, old valves and a worn out valve job... and they flowed:
Intake -- Exhaust
.1 063 -- 050.9
.2 119 -- 101.3
.3 168 -- 132.8
.4 201 -- 157.1
.5 210 -- 175
.6 218 -- 176.6
The intakes came out to around 155cc, and the exhaust 55cc. (1.94"/1.60" valves) They were just ported by 'eye', and seemed to work pretty well for what they were. This was done back in the days before the internet, right before all the aftermarket heads came out. They whooped up on more than their fair share of Chevies back in my late night street racing days.
For the exhaust port, I remove the 'step' and make the roof a straight shot to the back of the bowl. You lose a small amount of the guide, but it still works just fine.
Mine are C6 castings with the dog-leg exhaust port with the bosses made for the thermactors. I don't believe your C5 heads have that, and are probably more efficient.
I will also agree that 1.94"/1.60" valves are probably not optimal on these little heads. The 351W size of 1.84"/1.56" (I believe, that's the measurements) fits better. The bowl is getting really thin with the bigger valves, and the short side turn seems to get compromised.