The boat-tailed grackle's deep-V tail would seem to cause a great deal of drag or even downward deflection (like an airplane's elevators). How much does this actually affect the bird's flight?
I don't know of studies specifically of boat-tailed grackle flight, so I'll focus on elongated tails in general and come back to grackles at the end.
Long tails in birds are obviously interesting from the standpoint of sexual selection. Going all the way back to Darwin, one hypothesis has been that females prefer males with long tails. There seems to be evidence for this, although experimental reduction or elongation of tail feathers is perhaps of questionable success. See this back and forth in Behavioral Ecology.
Those interested in the study of flight have also studied elongate tail feathers to get at the questions you ask: are long tail feathers aerodynamically costly?
Balmford (1993) compared the aerodynamic properties of different tail shapes. He found that:
A forked tail that has a triangular planform when spread to just over 120° gives the best aerodynamic performance and this may be close to a universal optimum, in terms of aerodynamic efficiency, for a means to control pitch and yaw.
But the results were more complicated. Forked tails are only used in extreme maneuvers, they are more unstable, and suffer from greater loss of performance when damaged. He leaves open the possibility that natural selection, rather than or in addition to sexual selection, has played a role in shaping bird tails, possibly for increased aerodynamic performance.
More relevant here, Balmford et al. (1993) studied pairs of species, one with a long tail and one without, to assess the aerodynamic cost of long tail feathers. They studied ~600 specimens and used aerodynamic modeling to predict cost. Their results are summarized in this figure:
Graduated (rounded) tails are costliest over all lengths. Forked tails are notably less costly over all lengths, and shallow forks more efficient than deep forks (but there the difference is not great).
Our model suggests that as long forked tails deepen, it becomes more likely that they are aerodynamically costly..., and therefore increasingly probable that sexual rather than natural selection explains their evolution.
And of the species pairs:
... then among species with long forked tails, sexual dimorphism in tail length should increase with fork depth. Pairwise comparisons of closely related species support this prediction.
From what images I could find, it looks like boat-tailed grackles appear to have graduated tails when spread. If so, then these tails would be very inefficient and males with longer tails would incur a proportionately higher cost of flight than males with shorter tails.
Balmford, A., A. L. R. Thomas, and I. Jones. 1993. Aerodynamics and the evolution of long tails in birds. Nature 361:628–631.
Thomas, A. L. R. 1993. On the Aerodynamics of Birds' Tails. Philosophical Transactions of the Royal Society of London B: Biological Sciences 340:361–380.