At 3x yes, it's impractical. At 10x it's just barely practical. At 50x we'd be silly not to.
If we're 10x or 50x faster (at a steady, sustainable cruising speed that can be kept up for months and years, mind you) then the Kadeshi mothership should stick around Sol while we refit their warp engines. If it can be done in less than 18 years, it would be the smart move. Assuming everyone is showing good common sense, seems unlikely.