If you're talking about .17 Rem, rather than .17 HMR, I'm not sure what you'd gain by horsing around with .44 Mag brass. The .44 brass is a little wider, but also a little shorter. .17 is .3759" wide through the body of the case, .44 Rem Mag is .457". The .17 case is 1.53 to the base of the neck, the .44 is 1.28. When you allow for the capacity lost in necking down, you might, I repeat, might see a net gain of 4 grains, or so. Overboring isn't the most efficient way to gain performance, either.
If you've really got a hankering for ballistic nonsense, try making a .17-06. Or even better, a .17 Rigby.