Move legend title closer to legend keys

Question

I have the following plot:

library(grid)
library(ggplot2)

p <- 
  qplot(1:10, 1:10, color = letters[1:10]) + 
  theme_grey(base_size = 24)

But I'd like the legend title to be closer to the legend keys. I know I can just white-out the legend and overwrite it using grid.text:

p <- 
  qplot(1:10, 1:10, color = letters[1:10]) + 
  theme_grey(base_size = 24) + 
  theme(legend.title = element_text(color = "white"))

grid.newpage()
grid.draw(p)
grid.text(x = 0.87, y = 0.7, label = "LETTERS")

But this approach has the disadvantage of being inflexible.

I also tried adjusting arguments to theme(legend.title = element_text(...)) such as vjust = 1 but that had no effect (apparently a known issue).

Is there a better/canonical way to modify the legend title position?