Ported barrels in airguns? I think they must have a purpose, so I'm surprised that they haven't been tried more in FT rifles. The higher end Steyr pistols have three barrel ports, plus a small stripper cone. I don't think Steyr would bother with the extra manufacturing costs if they had no effect.
I can only think of two potential effects of such porting:
1. an upward blast of air to reduce muzzle climb.
2. reducing the pressure behind the pellet to reduce muzzle blast and potential instability; i.e. what most people think of as the purpose of a stripper.
I don't know what the effect of 1 is in the real world but there is a significant upward air blast from my LP10. On a cold day with a high dewpoint this can be seen as a puff of vapour 6-9 inch high.
If 2. is the purpose, it begs the question, why add a stripper cone too? Maybe the cone is more to do with dispersing the air in front of the pellet, letting it exit into "clean" air.
Taking it to extreme, porting could reduce the air pressure behind the pellet to atmospheric, letting the pellet effectively slide down the last few mm of barrel with no force behind it, but still being guided by the rifling. No air blast behind the pellet at all. Logically shouldn't this be the scenario with the highest accuracy?