I'd go with size 16, yellow, no outline, black drop shadow (drop shadow shouldn't be "soft," and it should go down and to the right), and probably a Franklin Gothic or Helvetica-style font. Also, a wee bit higher.

And I'd also preserve the color changes as present on the Region 1 DVDs (as in, in that one scene they're green, and then red, instead of yellow), but also keep the same size/font and black drop shadow.

I think Inglourious Basterds is a good reference for Kill Bill subtitles. They kept the theatrical (burned-in) subtitles on the DVD/Blu-Ray (I think), and they look pretty much exactly how I remember the Kill Bill subtitles looking in theaters:

(totally didn't realize that was a GIF until I pasted it in here, haha)