Tayshaun Prince-Josh Howard; Peja Stojakovic-Jerry Stackhouse; Kevin Martin-Gerald Wallace; and Rudy Fernandez-Randy Foye: those are the starting SF-SG tandems that Garcia and Dunleavy will have to face within their own division. I don't see how they'll be able to keep their opponents from scoring.
I'm not sure that Peja/Stackhouse and Fernandez/Foye are clearly better than Dunleavy/Garcia, especially when you take injuries and age into account.
I doubt Peja will have much of a problem getting his shots off vs. either Dunleavy or Garcia.
True, Fernandez and Foye aren't much better, but both are quicker than either Dunleavy or Garcia, both of whom ought to have a hard time staying between their man and the basket. Foye, for instance, ought to repeatedly toast whoever draws the short straw.
I think you're underrating Dunleavy. Why do you think he's a poor defender? He's quick, has good lateral movement, stays in front of his man... He's quick enough to defend SGs, and he's tall enough to defender SFs. He's also a very good rebounder, an excellent shooter, and a skilled passer. Seriously, the guy is a good player; I don't understand why folks get on his case. I think it has a lot to do with 1) where he was drafted, 2) the college he graduated from, and 3) his appearance. It's certainly not based upon his play on the court.
Mike Dunleavy was a much better player than Peja last season, and frankly, it's not close. He had significantly more points, more rebounds, more assists, more steals, more blocks, shot a higher percentage, and had essentially an identical eFG%.
Garcia is similarly better than Stackhouse, almost across the board. He scores more points, has more rebounds, more blocks, more steals, and shoots *significantly* better, both overall and from three.
Dunleavy/Garcia destroys Peja/Stackhouse, and that's without considering injuries and age.