So @Arran I've been doing those tests with someone else https://replay.faforever.com/14013744
So I started with 5 swifties vs 20 inties, which is balanced costs more or less (1175 M / 33,7k E / 9k BP // VS // 1000 M / 45k E / 10k BP) and got crushed, then moved to 6-7-10 swifties to see what happens.
Conclusion N°1 : micro is the most relevant factor (unsurprisingly). I lost most of my fights cause grimplex just turned better and shot with his planes more often.
Conclusion N°2 : you can hit and run. I attempted this at fight n°2, instead of making a circle, i just fly into the ennemy planes and fly away. I don't lose any swifties while he loses a few. This can certainly be improved (i could have taken a better engagement) but in theory i could repair them and fight again later.
Conclusion n°3 : in prolonged fights with even micro, swifties do turn faster and can end up right behind the ennemy planes, where they can't shoot back (around 6min in the replay we take 1 or 2 of those fights).
Overall : i'd need more tests to be sure ( with better micro from my part ..), but as you said, swifties are not mass efficient at all against inties. You can find some ways to get an even fight, but you can't win convincingly mass for mass.
That being said, they are still faster which truely helps in every situation, and allow you to pick the fight you want and avoid the ones you don't want to take.