My apologies if this has come up before, but how effective is Tarn plus the R7 astromech? If I am reading it correctly, as soon as someone targets him he gets to acquire a target lock on the attacker, then with R7 he can make the attacker reroll any hits, which cannot be rerolled again. It seems like this strategy can even be used multiple times in a round. Since I've never really seen anything about this, does that mean it's not that effective?

yes its a fun combo...not super effective because of the limitations of the X-wing

So your standard TLT Ywing is a more effective use of 25 points?

yup

until they fix the x-wing

Tarn + R7 is a good combo. However, while Tarn can use his ability an unlimited number of times, the R7 text specifically states that it can only be used once per round. I don't think TLT y-wings are inherently superior. Tarn is PS3, has an ability that helps with action economy (you don't always have to use it defensively) and R7 means that he is annoying to kill (so given a choice, your opponent will prefer to shoot someone else in your squad). Tarn, along with all T-65 x-wings, will get decidedly better once wave 8 comes out because of the new integrated astromech modification (which should be put on any x-wing carrying a droid since its essentially a free shield upgrade....unless you really want engine upgrade or perhaps experimental interface).